Dentists' rhymes with:
dentist's, dentist's, adventists, dentist's, scientist's, scientists, scientists', adventists, chartists, clarinetists, cultists, dentist's, elitists, hypnotists, leftists, pragmatists, rightists, scientist's, scientists, scientists', separatists, statists
Dentists' sounds like:
dentist's, dentists, donatists