There are scans and images for most official cards online. Usually you can compare to those, and if something is off you might have a fake. I'd also look into the quality of the card -- so the colours may be printed poorly, or the card may be flimsy or weak. Keeping a set you know is real as a reference is helpful.
I'm by no means an expert, but I've come across my fair share of fake cards and these were some of the things I noticed about them. Someone with more experience will probably have better advice.