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]
Words nearby be on to
be in on,
be into,
be my guest,
be off,
be on,
be on to,
be oneself,
be prepared,
be that as it may,
be the death of,
be the end of one