Nothing. You can't tell. Unless you find actual hard evidence like a police badge or receipts for payments or something. Go read up about all the hilarious fuck ups that the FBI made trying to sus out cold war spies. Read about the people they falsely accused, and the highly placed spies they never found.

You're not the FBI. You don't have the resources to conduct mole hunts. Unless someone breaks in to a cop department and steals all their cop documents you'll likely never know who is and is not an infiltrator.

Just be zen about it. Do your opsec as best you can, assume that you're compromised, and make peace with the consequences.