A little light reading. Say thank you to the State of New York and Academia.edu, who made the linked download possible. (You don’t need to sign in or sign up to download, but if you’re worried about it you should still use Tor/VPN/standard tradecraft.)
In order to defend against a tactic, you need to understand how the tactic is done. Pay special attention to Rule #9 of informant handling, because it’s a list of “signs of duplicity” they look for when handling an informant. I’m sure you can figure out which items would be helpful for you and which would not.
The same things they look for in an informant are the things you should be using to pick out who already is one, or who is most susceptible to being flipped. Look for people who have “beef” with others; that will be an avenue they use to drive wedges. Look for people who are easily flattered—often when opening a conversation, agents will compliment their subject and attempt to break the ice or get the subject to let their guard down. Think through the members of your group: if approached, how cooperative/confrontational would they be?
Look for disparities: Does the subject work at a low paying job but have a nice house, cars, etc.? Do they claim to have a master’s degree but speak like they’re fresh out of the trailer park? Do they claim to have military experience their skills don’t back up? Do they claim to be from somewhere but can’t answer questions about the area?
The document above is for New York State, but it was written by someone with 25 years of undercover with federal agencies. Take, read, apply what you need.
And don’t forget Rule #1: Don’t play stupid games with stupid people…or smart people pretending to be stupid.