be on to someone

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 on to someone

be close to discovering the truth about an illegal or undesirable activity that someone is engaging in. informal
See also: on, someone