To put your finger on something

To put your finger on something - to discover the exact reason why a situation is the way it is, especially when something is wrong.

Example - "There's something odd about him but I can't quite put my finger on it"

Image - http://www.quotehd.com/imagequotes/authors28/mike-ireland-quote-i-cant-put-my-finger-on-whats-the-problem.jpg