Physically, what makes it possible to distinguish a Neandertal from an anatomically modern human?
Neandertals are smaller in stature than their modern human contemporaries and they were more robust. Their cranial volume was also greater. Although this is of course all relative.
When examining a fossil, in order to be able to know with certainty whether we are dealing with Neandertal bones, we must see a combination of derived characters, that is to say, their own characters, which are the result of their evolution and which are not found in their ancestors or in anatomically modern humans. It is the sum of these observations that gives us a sure diagnosis.