put (one's) finger on (something)
1. To recognize or recall fully, clearly, or correctly. (Usually used in the negative.) I can't put my finger on what this is reminding me of. There's something off about her story, but I can't put my finger on it.
2. To recognize, identify, or determine the fundamental aspects of something. Well, I think you put your finger on it when you brought up the issue of tax increases for lower earners.
put one's finger on
Also, lay one's finger on.
1. Identify, as in I can't put my finger on the man in that photo. [Late 1800s]
2. See lay a finger on.