be on to
Be aware of or have information about, as in They can't pull that trick again; we're on to them now. [Colloquial; second half of 1800s]
Discover something important or profitable, as in The researchers claim they are really on to something big. [Colloquial; mid-1900s]
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.