Usually yes. I can always tell a difference between a Japanese and a Chinese, mostly in their attitude and behavior, not the face. When you see an Asian person trying to push ahead of you in line, you can be sure he ain't from Japan.
In regards to physical appearance, Thai and Vietnamese to me have doll like faces (smaller lips and eyes).

No problem.  Japanese, Korean, Chinese, Thai, Vietnamese, and Philipino people all have distict facial features.  It's hard to describe how, it's just something you learn over time.
Growing up in a very ethnically diverse city probably helps.
I can usually even tell the difference between Hunan, Hubei and Guangdong women (men not so much).

Yeah, it's not hard really. Koreans often have wider faces and are more.... curvy. Japanese are usually the shortest. Chinese, due to the country's geography, come in all shapes and sizes and colurs. 

Can you tell the difference between white people?

Yes, it's not difficult.  Mattaya, do you have a link to a larger version of that picture?  I'd like to check it out.  I can't read it because it's too small!

Chinese girls,have knickers at the bottom of their pants

I think they all look like clones, just like all us white people look the same. You can't pick them out in a crowd unless you remember what they were wearing. But I'm near sighted and don't where my glasses, so this may be different for you.

You can tell the difference between Koreans, Chinese, Japanese, etc. by looking at them just as easily as you can tell if someone is Irish, English, French, Russian, or German just by looking at them.