If you hold a card and it feels light, or otherwise weird, odds are it's not genuine. I just got an ultra rare no foil Charizard. How many times have we all heard that. You all know what that means it means that you just got ripped off. Sometimes counterfeiters are so lazy they don't even bother to put a fake foil finish on the cards.
Foils are foils, that's all there is to it.
If there is a card that is only found as a foil, and you have a non foil version of it, it's probably counterfeit. Here's why you have to destroy the card to prove it's a fake. As an extra security measure, cards produced by Wizards of the Coast have a glue like substance in the middle. It's what gives the card its flexibility and its thickness. This is a very expensive feature, and most if not all counterfeiters will not bother to duplicate it.