Get/take/have the measure of somebody
Definition 1 of 1
Pro Tip 1/3Judging Character
The phrase is often used when someone is trying to judge another's nature or intentions. 
The detective took the measure of the suspect calmly.
Pro Tip 2/3Professional Assessment
In work contexts, it relates to understanding a colleague's or employee's work capabilities. 
Give me time to get the measure of his negotiation skills.
Pro Tip 3/3Competitive Edge
Used in sports or competition to describe learning about opponents to gain an advantage. 
Our coach has taken the measure of the rival team's strategy.