If you see a chip inside with an MX logo on it, it's time to get wary. If you see one or two or sometimes even four windowed eproms (SS3/KOF95/MOTW for instance) there is no need to worry, but when you get a cart where every chip is windowed, it's a boot. Small surface mounted chips mounted onto adapters and then soldered to the boards is a dead on sign of bootlegging.
To find an original, look at the chips inside the cart. If you find a number on the chips that matches the master list on this site, you are safe. For example SS2 is #063, so if you open an SS2 cart and find a full set of chips stamped 063, your're safe. This even applies to a cart with one or a few windowed eproms. Those ones are usually SNK after production fixes.
A fake label or a board with an SNK logo means absolutely nothing. There are many many real carts with a fake english label slapped over the original Japanese one as well as many many carts with original SNK boards but with swapped chips that are fake.
If you stay away from MX-chips, adapter mounted SMD-chips and full sets of windowed eproms, you are generally safe. It's not that difficult, really..