(verb) to take a reading is to check out a particular situation in a small way, in order to get an accurate idea of someone's character, usually for the future when the stakes may be higher. The person who is curious may do this deliberately -- or accidentally, for example by observing how a certain person treats others.
Friend #1: "Should I loan Alex my car?"
Friend #2: "Didn't you take a reading last year, when he borrowed your bike and returned it all beat up?"
Friend #3: "Yeah, David definitely seemed like the man of my dreams, but I had to take a reading when he was rude to our waitstaff."
Friend #4: "OMG, how pathetic!"