It's hard to say without actually speaking with the girl, but I can try.

Can she simply not hold a conversation in English, but has a few words already? Look for a textbook marked 'beginner'.

Can she speak using full sentences, but with constant mistakes? Do you need to speak slowly and using simple language to be understood? Does she often struggle to find the word she's looking for or often find the wrong word entirely? Does she usually speak about everything like it's happening now, even events from the past or planned for the future? Does she often answer a question like you actually asked a different question (eg 'how are you/yes, it's nice to see you too')? Then look for a lower level textbook, usually marked 'elementary. From your examples, this would be my best guess as to what's appropriate.

Does she have the ability to speak with you without too many pauses (although some)? Does she use some different verb tenses to show differences in meaning and usually get them right (for example: go, went, been, going to)? Do her responses to your questions or comments usually make sense in terms of the question asked? Then look for a mid-level text book (often marked 'pre-intermediate' or something similar).

Can she speak on a subject for a couple of minutes without struggling to find many words? Can you speak to her without having to slow down too much, or repeat yourself? Can she usually find the right word to express what she wants to say? Do her more complicated sentences still make perfect sense, with her more basic sentences almost always perfect? The look for a higher level textbook (usually marked 'intermediate').

There are higher levels, but based on the examples you've given of her language usage, I don't think they'd be appropriate. 

If you're not confident selecting a book yourself, then maybe try to make friends with a competent English teacher.

Are you living with her?  My gf's english improved exponentially after only a few months together.

Label everything in the house by what its English name is, and have her say them any time she wants something or talks about something. Also sing a lot with her and have her sing everything she says. We used to have to teach the Brazilian students that way back in school.

as long  as she can say yes then her english is good enough

What MissAustralia2012 said!

But, if you're talking about reading there's plenty of good ESL sites where she could do an online test, to get a general idea.

But, OTOH, don't worry too much about it! My best (male) Chinese friend here hated English, kept failing, even had a couple of shots at CET4... and failed. (maybe only barely passed in compulsory school and uni english). He met me about 2 years ago (to teach me kungfu). At first, we needed a translator. After 3 months, we didn't, but it was slow. We kept heading out for dinner etc after training, met up with other teachers for drinks etc, etc etc... now, he's working for a company in Shanghai because of his English (he's basically a translator/salesman for overseas buyers).HE DIDN'T STUDY for this! We just chatted! He didn't go reading books...

Now, he also loves English! And he's trying to learn Spanish as well. He found a good reason to get better... it's no longer about 'studying some useless thing', but a way to communicate, to have fun and laugh with different people from all over the world!

So, tell your girlfriend not to worry about it. She learnt Chinese just by listening and talking... she'll do the same with English!

Ask her.....

How much wood would a woodchuck chuck if a woodchuck could chuck wood ?

Hi, The answer to the question depends partially on why you need to know the level of her English. There are specific ways of measuring the ability through. I dont know if the following will help you

Assessment of English Language

Levels
 

One of the aims of the "Common European Framework of Reference: Learning, Teaching, Assessment", Council of Europe reference document for the European Language Portfolio, is to help partners to describe the levels of proficiency required by existing standards, tests and examinations in order to facilitate comparisons between different systems of qualifications. For this purpose the Council of Europe has developed a European Framework with common reference levels.

There does appear in practice to be a wide consensus on the number and nature of levels appropriate to the organisation of language learning and the public recognition of achievement.

These six levels are an interpretation of the classic division into basic, intermediate and advanced.

The scheme proposed in the "Common European Framework of Reference: Learning, Teaching, Assessment" adopts a "Hypertext" branching principle, starting from an initial division into three broad levels:

Basic User: A1 and A2

Independent User: B1 and B2

Proficient User: C1 and C2

Such a simple "Global Scale" makes it easier to communicate the system to non-specialist users and will also provide teachers and curriculum planners with orientation points.

However, in order to orient learners, teachers and other users within the educational system for some practical purpose, a more detailed overview is necessary. Such an overview is presented in the form of a Self-Assessment Grid showing major categories of language use at each of the six levels. It is intended to help learners to profile their main language skills, and decide at which level they might look at a checklist of more detailed descriptors in order to self-assess their level of proficiency.

Global Scale

Proficient
User

C2

Can understand with ease virtually everything heard or read. Can summarise information from different spoken and written sources, reconstructing arguments and accounts in a coherent presentation. Can express him/herself spontaneously, very fluently and precisely, differentiating finer shades of meaning even in more complex situations.

C1

Can understand a wide range of demanding, longer texts, and recognise implicit meaning. Can express him/herself fluently and spontaneously without much obvious searching for expressions. Can use language flexibly and effectively for social, academic and professional purposes. Can produce clear, well-structured, detailed text on complex subjects, showing controlled use of organisational patterns, connectors and cohesive devices.

Independent
User

B2

Can understand the main ideas of complex text on both concrete and abstract topics, including technical discussions in his/her field of specialisation. Can interact with a degree of fluency and spontaneity that makes regular interaction with native speakers quite possible without strain for either party. Can produce clear, detailed text on a wide range of subjects and explain a viewpoint on a topical issue giving the advantages and disadvantages of various options.

B1

Can understand the main points of clear standard input on familiar matters regularly encountered in work, school, leisure, etc. Can deal with most situations likely to arise whilst travelling in an area where the language is spoken. Can produce simple connected text on topics which are familiar or of personal interest. Can describe experiences and events, dreams, hopes & ambitions and briefly give reasons and explanations for opinions and plans.

Basic
User

A2

Can understand sentences and frequently used expressions related to areas of most immediate relevance (e.g. very basic personal and family information, shopping, local geography, employment). Can communicate in simple and routine tasks requiring a simple and direct exchange of information on familiar and routine matters. Can describe in simple terms aspects of his/her background, immediate environment and matters in areas of immediate need.

A1

Can understand and use familiar everyday expressions and very basic phrases aimed at the satisfaction of needs of a concrete type. Can introduce him/herself and others and can ask and answer questions about personal details such as where he/she lives, people he/she knows and things he/she has. Can interact in a simple way provided the other person talks slowly and clear

Kind Regards Mike

My GF bought a ''good'' book to teach her reading. She can't read it, so I think she needs to start  basic. She says maybe I can't teach her.

Aaaah, I tried to post a comment and it came out an un-paragraphed mess, so here goes a second time...

Maybe you can teach her and maybe you can't; to teach effectively is nowhere near as easy as commonly made out.

At the same time, if she's self-evaluated and decided she knows her current level and abilities, ignore it. DON'T listen to your girlfriend saying she knows what learning the English language involves and what she needs, and that by following her orders, if things don't work out it's your fault. Just... don't!

Buy your own beginner's book if you must, and say that of course her book is great and you're sure it will be very useful down the line, but this one suits you and your style better at the moment. And good luck. Do remember that whatever else happens she's communicating on a regular basis with you, using English. As shining says above, it's likely she will dramatically improve based on just that fact.

Failing that, get her lessons with an actual, qualified and preferably experienced teacher (hint: age, gender and race are not qualifications. A native speaker is great to help her with with pronunciation, but do check they know and can explain what a dipthong is, and can explain the difference between an adverb and an adjective, that's the bare minimum you should expect for someone who wants to get paid to teach adults privately).