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.
Ask them what job they have. If they respond with "CIA operative", they might be an infiltrator
Do they have above-average driving skills?
Biden's dad = confirmed cop
Seriously, though, great content in these links.
Some of the general stuff is like,
Are they advocating for escalating violence?
Are they spreading rumors about other members?
Maybe if they stall things they are supposed to be working on for a blatantly long period of time? The old CIA guidebook that got shared a little while back mentioned that https://www.openculture.com/2015/12/simple-sabotage-field-manual.html
The key is not to make sure everyone in your organization is trusted, but to make sure there's a culture of the organization that follows a code of conduct that prevents domination or wrecking. Also a culture of opsec where the secrecy of an op determines how widely the details are shared. As long as this is followed, the cops can send as many folks as they want, and we'll put them to work building socialism while giving them absolutely no intel.
No way, there was an internal memo thing from police shared a while back here about there failure to infiltrate anarchist organizations because the theory was way too difficult for them (although I think this was local, we have to assume the FBI is more successful and it's the successes you don't hear about that are the problem). Censors in Russia allowed Capital to pass freely because they couldn't grasp the dry economic analysis leads to a necessary outcome of revolutionary politics. Prager U recently said "[...] few, if any, will suggest Marx’s 'Das Kapital' because almost no leftists have read it and because you will either not finish the book or reject it as incoherent."
Treat everyone like a fed but also read Das Kapital
ok right let me amend that "read all of kapital and refer to it in a way that makes you feel they understand it passably to very well*"
Posting this in order to find out how not to look like an op
spoiler
I'm joking, I'm joking
Accusing others of being something worse than a FBI/CIA informant- a lib.
Check their shoes, man! 😵
No, wait. I might be thinking of undercover narcs
