get someone's number

Also, have someone's number. Determine or know one's real character or motives, as in You can't fool Jane; she's got your number. This expression uses number in the sense of “a precise appraisal.” Charles Dickens had it in Bleak House (1853): “Whenever a person proclaims to you, 'In worldly matters I'm a child,' ... that person is only crying off from being held accountable ... and you have got that person's number.” [Mid-1800s]