How can you tell? I probably don't want to know, cuz then I might have to find out I have em.
If your carts don't have the sticker like Q*B stated or if it's been broken, the only real way to make sure is to open the cart and check the chips. The chip numbers should match the NGH numbers is listed
here. If you have an EPROM it's not necessarily a bootleg, but if you have MX flash chips it most definitely is.
Anyway, back on topic... Yes, I've done the same thing with coin doors. I payed a pretty penny for my first and then they start falling from the sky.