I'll be honest, I just read most posts and think "how does this read to a normal human being?" If it comes off as really antagonistic, divisive, or otherwise unhinged, I usually put that person on my sus list. Doesn't always mean I think they are feds outright, but there's some people here that have made some really, out of the blue, crazy comments from a seemingly innocent question or discussion and I immediately put that person on the sus list. A few of them I just don't respond to any of their posts simply because they seem like bait to try and garner personal information. I don't want to call out posts or names though.
This probably isn't super helpful and I'm sorry if that's the case it's just, you kind of develop an eye for it. Like between the legit leftists I listen to podcasts of and the real communists I talk to regularly, they are almost all just very normal, logically minded humans, with normal everyday thoughts, desires, etc. So when I see stuff posted and thing "if my buddy Joe read this he'd think I was fucking crazy for coming here" then that person probably needs to touch grass or glows in the dark.