from my experience there are no fake bitcoins, all i know is altcoin but the price is still far different from bitcoin price. If you have ever met fake bitcoin, then you have to be careful in making decisions if you choose the wrong choice then you will get a big loss.
Most of the fake bitcoins are those that forked from original bitcoin and attached the name of bitcoin to their name. And they considered themselves as replacement for original btc, but they are not. Just another alt trying to steal the fame from btc but they couldn't.