be onto somebody
be on to (someone)
1. To have knowledge, be suspicious, or be aware of someone's actions, behavior, or intention. You'd better tread carefully, I'm on to you now. The police are on to us! Everybody run!
2. To be in contact with someone at a given point in time. I'll be on to you tomorrow about the details of our meeting. I was on to John yesterday about where he wants to go for his birthday.
See also: on
be ˈonto somebody
1 (informal) know about what somebody has done wrong: She knew the police would be onto them.
2 (also get ˈonto somebody) be talking to somebody, usually in order to ask or tell them something: They’ve been onto me for ages to get a job. ♢ I must get onto the local council about all the rubbish in the street.
See also: somebody