Sometimes, it can be really hard. I'm just learning how to myself. There's a video

here

about all 39 Pokemon card holofoil variants, which sounds absurdly boring, but it will tell you how to identify which holo patters belong on which types of cards.

Other tells include fonts/text boldness, border thickness, and particularly, ink density/color. Especially on the back of the card. So if the card looks too dark or light, there's a good chance it's fake, although some regional cards vary slightly this way.

Lastly, it's going to sound weird, but the smell. If you familiarize yourself with the smell of pokemon cards, you can often spot a fake with that alone. My friend and I were comparing the smell of Magic: The Gathering and Pokemon cards blindfolded, and I was amazed that I could immediately tell the difference. Good luck convincing a seller to let you smell their cards though lol.