Welcome to the boards
There are many ways to tell if a game is a fake/convert.
1) Fakes are mostly the rare/expensive homecarts.
2) There is no manual for the game.
3) The insert is diffrent from the original.
4) The cart sticker itself or manual is slightly diffrent.
There have been some well made conversions out there....pretty much the only way you can tell if the game is original or not...is to open the cart & take a look at the boards.
If your looking to buy rare/expensive homecarts i would personally recommend avoiding Ebay & deal with reliable members on the boards.