FaceDeer

Some years back I was in a D&D campaign where doppelgangers became a major ongoing concern. It turned out that in that case doppelgangers built up their image of the person they wanted to mimic through careful observation, but thanks to the general prudishness of society doppelgangers rarely ever caught glimpses of peoples’ genitals. So we ultimately came up with the “crotch check” system. Doppelgangers usually couldn’t form plausible genitalia.