1
general::
Become aware of, notice, as in The teacher picked up on her nervousness right away. [Colloquial; mid1900s]
American Heritage Idioms
2
general::
Phrase(s): pick up on something
to become alert to something; to take notice of something; to learn or catch on to something. • She’s real sharp. She picks up on everything. • The cop picked up on the car with the expired license plates.
McGrawhill's American Idioms And Phrasal Verbs