Not to encourage people to do this, but there are vendors that specialize in creating stamp reproductions of any stamp, and they are notated on the backs. Unfortunately, there's nothing to keep others from re-selling these without mentioning the notation, or even removing the notations.
Unlike the old master forgers, there's no book/archive for itemizing these modern reproductions/counterfeits -- you simply have to slowly gain philatelic knowledge over time and not only know how to spot them, but actually take the time to notice. It's becoming a real waste of time, but now an unfortunate necessity. I've noticed a non-trivial increase in the number of forgeries/counterfeits/reproductions in lots I've purchased from established reputable dealers over the years. You can't expect them to check over every single stamp in the lot. You buy lots hoping for finds from what the dealer may have missed -- but it goes both ways, and now starting to find more and more fake stamps.