sex vs tell apart

sex is a noun but tell apart is not a noun.

Word	Noun	Adjective	Verb	Adverb
sex	Yes	No	Yes	No
tell apart	No	No	Yes	No

As verbs, tell apart is a hypernym of sex; that is, tell apart is a word with a broader meaning than sex:

Other hypernyms of sex include differentiate, distinguish, secern, secernate, separate, severalise, severalize, tell.

sex (noun)	tell apart (noun)
activities associated with sexual intercourse
the properties that distinguish organisms on the basis of their reproductive roles
all of the feelings resulting from the urge to gratify sexual impulses
either of the two categories (male or female) into which most organisms are divided

sex (verb)	tell apart (verb)
tell the sex (of young chickens)	mark as different
to evoke sexual feelings	detect with the senses