how can someone differentiate between the original and the fake yuan in china?

The easy way is to put it into the money detector. If you don't have it, you can touch the both sides of the Yuan. If it's very thin, it might be fake. Or you can look at the denominatino ID in bottom-left of the front side of the Yuan from different angles. If the color changes, then it should be a real one.

I might have some other useful tips: If you can't tell if the 100, 50 or 20 RMB is real or fake, just ask him/her to give you the 10 yuan or 5 yuan changes. Be very cautious if someone gives you very dirty and old yuan or a brand-new note.