Not an expert but from my experience so far if the sticker is perfectly white compared to the plastic body which is yellowed in comparison then that is one giveaway, because the sticker should have also slightly yellowed too over time. Another is seeing visible printing lines in the design from a standard home printer, also if the sticker surface is matte/dull then it's printed with low quality printer paper, an original should have a slightly gloss sheen to the surface. The colours are also a dead giveaway on repros, if you look at the blue's compared to an original sticker they're usually a different shade.
However I have seen a couple of repro stickers recently that actually look quite good on ebay, but again it would come down to having experience and spotting the difference, owning an original to compare would def help.
You don't own your toys, you only borrow them until you sell them or die!