Sometimes they do bring in actual teeth and sometimes in quantity. Back when I worked for a coin shop I just learned after a point to fill out the paperwork and follow standard-work for said item. One guy — I just had to ask if he was a dentist and his reply was “No ---- Marine”. I never asked after that. Yeah, there really are people like that around but not as much as in the past.
For us it depended on the bar; size, origin, and other factors. People fake commercial/minted bars just as they do coins and sometimes bars have odd origins being several sources of material melted together by amateurs or small-shops making something “Gramma can hide easier”. Our usual thing was to have it smelted and assayed and taking a fee and a percentage for our effort. That usually got the seller the best return as our risk was minimal and the base charge (from the smelter) would be covered by the “obvious gold content” we had already confirmed present. One big win was some small (6-8 ounce) bars of gold over platinum from an era when platinum was much cheaper than gold. In a way they were very old “fakes” but in terms of melt a great win for the owner.