The adjective discerning means someone who can judge well if something of a particular kind is good or bad.
Synonyms are insightful, knowledgeable, knowing, or astute.
The word origins from 1600, present-participle adjective from discern in the sense “discover by the intellect, understand”.
I admire his discerning instinct.
He is a discerning reader.
We trust his discerning eye to pick the right candidate for us.