I'm not going to go through all the fakes as I know there are several guides out already on them, I am however going to write about a game boy game that I found to be fake. I bought 2 Pokemon Gold games from 2 different sellers, and soon realized one was a fake. I started to worry when the seller was kicked off ebay that I wouldn't get my game, but after I proved to him that I payed him he sent the game and everything seemed fine. THIS GAME DOESN'T SAVE!!! That's not the only problem though, after taking time to compair the good working version to the fake, I saw differences in the Picture on the front. While everything looks smooth on the real working game, the fake is pixilated and actually in some spots shows bits of Ho-oh's wing through the Pokemon logo. Also in the right lighting the grey bars on the front sticker are noticably darker. Another good thing to look at if you think you have a fake is The gold nintendo logo. The one on the fake is noticably faded and hard to read, while the real logo should be easy to read. (If you have good vision that is, lol)   This is a pic of the fake, where you can see that the Pokemon logo is not totally smooth and clean, it's broken up a bit and in the dash above the e you can see a gap in the color. Just to show you how the real one looks, I'll add another pic.

  This one is more smooth and has no visible gaps that I could easily spot.

I hope this helps when you go to buy the game.