Gucci do not and have never made the following:
Stamped snake-skin, lizard, crocodile or alligator.
If someone has a bag that is a stamped exotic it aint Gucci
Faux-leather - Watch out especially for Tom Ford mid-late 1990s high-shine, polished leather examples especially in black (sometimes with GUCCI written in upper-case on the outside). It is
so hard to authenticate these bags unless you are an expert in this era. Lots of fakes on the market from that generation too because they were relatively easy to copy. Most copies are faux leather but there were leather copies too alas.