pick up on - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Become aware of, notice, as in The teacher picked up on her nervousness right away. [Colloquial; mid-1900s]