Have Someone's Number Meaning
(idiomatic) To understand a person's character, capabilities, or situation.
Example: 1922, Rex Ellingwood Beach, Flowing Gold, ch. 10:
"Say! He's the hardest guy I ever saw," Stoner declared, admiringly. Mallow spoke last, but he spoke with conviction. "You said it, Brick. I had his number from the start. He's a master crook, andâ€”it'll pay us all to string with him."
1994, Marilyn Stasio, "Crime" (review of One for the Money by Janet Evanovich), New York Times, 4 Sept. (retrieved 20 Nov. 2009):
Morelli has her number: "Lots of energy, not much control, sexy as hell."
2008, "Tennis: Murray edges to win over Llodra," BBC.com, 28 Aug. (retrieved 20 Nov. 2009):
Llodra, a doubles specialist who is ranked 38 in singles, was charging to the net at every opportunity and played some brilliant shots but Murray seemed to have his number at that stage.