Reminds me of the old joke. How do you know who the FBI informant is at the KKK rally? “He’s the one with the dynamite”.
This is pretty much common knowledge. The way you identify FBI informants is simply to look for the ones advocating violence. A militia is, by its nature, primarily a defensive organization. (as in "for the common defense") Anyone attempting to change that is most likely a government plant attempting to entrap or kill someone.