A lot. But most of the most important items on any list can be satisfied by actually visiting the place where you are going to teach and meeting the people you will be working with.

Check everything. If you know any lawyers then get them to read the contract. If the school tries to screw you it might scare them into straightening up if you tell them your lawyer will get involved. They'll usually back down since getting the court involved requires too much time and money, more than the average foreign teacher is worth to them.

With the technology available, you should check what is being said about the school online. You will come across the good and the bad comments, but you have to figure out what is more believable, and still take everything with a grain of salt. If you know people who have taught in China, talk to them. Ask lots of questions.

Compare their online ads/salary/perk offers (accommodations etc) with other schools in the same region. If one school is offering 10000rmb for the same work being offered in other schools that are only paying 5000rmb... probably too good to be true.

Ask to see pictures of the apartments they are offering, ask what comes in the apartment (furnishings, home office, kitchen stuff, bed linens, etc). Ask to speak with other foreign teachers that have worked for the particular school in which you will be teaching. Speak on Skype or other voice app if you can. Ask LOTS of smart questions. Let them know you aren't some idiot that will be happy with any crumbs they throw at you.

When you are confident in what you have selected, and finally show up, be prepared to roll with the punches. It probably won't be everything promised. Register any issues you come across immediately. Write them down for your own records. Keep at them if they don't get fixed. Just don't lose your cool right away.

Just for fun, search ESL in China on youtube. You can see video of some classes. It might help you decide what age group you want to teach.

Here are a few links to help in your investigating
http://www.tefl.net/forums/index.php
http://www.onlinetefl.com/tefl-chalkboard/
http://www.eslcafe.com/
http://www.eslteachersboard.com/cgi-bin/tesol/index.pl

The most important thing to do when considering a teaching offer is to get the Chinese version of the contract translated into your mother tongue from a source you trust. The only legal contract in China is the contract in Chinese, therefore it is very important that you make sure that the Chinese version of your contract that the company sends you matches the translated version of the contract that they provide. Besides that, what you read is what you get so it's up to you and your personal preferences. If anything is vague or you don't understand a certain aspect, get it clarified BEFORE you sign.

I wouldn't worry about it. As long as they get you a Z visa that is all you need to stay here. Nowadays China is quite modern and they are not into screwing you. Well I haven't been screwed, not to much anyway. Just come here and have a good time, learn Mandarin then you are set for the future. If you are not in China I would recommend a skype video interview so you know what you are getting into. I haven't heard any horror stories lately so i think things are pretty good at the moment.

gzpeterlee must be someone working at the school recruiting you because that is the biggest crock of bulgogi I have heard in a while.

Of course you should worry about it. For starters you are buying a presumably expensive plane ticket to come out here; that's more than enough reason to be concerned about if you'll actually like it and if it will be worth it.

There's a lot you could worry about ... I think the best advice if you're willing is to come on a 60-day tourist visa and apply for jobs here (it's perfectly legal). People will tell you anything in person, much more so on the internet. But at least if you're here you can ask around , visit the schools , talk to other foreign teachers directly , etc . Being here now and not being overly fond of my job , it's exactly what I WISH I had done . At the time, however, I really didn't realize just how many jobs there were here . I thought I had seen many job posts online beforehand ; it was nothing compared to what I've seen since I have actually been here .