be on to (something)
To discover, realize, or be in the process of doing something of great importance, value, or insight. Great work on this essay, Lindsay. I really think you're on to something here. Scientists now believe they may be on to a cure for cancer.
See also: on
To be in the process of finding or understanding something: I'm not sure, but I may be onto a solution to this math problem. The gang was afraid that the police were onto them.