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be on to

be on to

 
See:
on
part of speech: phrase
definition 1: (informal) to have inside information about.
"Look," said the detective to the suspect, "I'm on to you, so don't think you're going to get away with this."
definition 2: to have an accurate suspicion, theory, or prediction about something.
She knew she was on to something when her officers began collecting more and more evidence.