Carter Page was supposedly in the employ (in some fashion) of the FBI just months before joining the Trump Campaign.
How do we know he wasn’t still in the employ of the FBI while in the Trump Campaign?
We don't...yet. Actually, when that rumor about a spy in the Trump camp was circulating, I thought it was Carter, not Papadopolous (sp?). That's because Carter was working for the FBI in some capacity late in 2015, IIRC.