I misidentified pretty embarrassingly once; I'm much more careful now than before as a result. But there's a few things.

1. A big tell is when people who have never been seen in here before suddenly Have Opinions™. That sparks immediate suspicion. It's not conclusive, but it's a flag.
2. An even bigger tell is that the Opinions They Have™ seem to take the man's side, or use some argument about "human nature" to put the men in the right and so on.
3. A lesser tell is there is some wording issues that raise a smaller flag for me.

But once the tells are there and I start looking, the first place I look is their posting history. A lot of them just flat-out state that they're dudes in some post or another. Others will mention their wife in one post, then in another post say they're cis-het. If you're cis-het and you've got a wife, you're pretty much automatically a dude because of, you know, what 'cis' and 'het' mean. Also a history of commenting on men's issues is a big tell.

And then, if I'm VERY suspicious but still not really certain, I hit the mod log for the user. And there it often comes glaring out. Even if they don't identify as male in the mod log entries, the nature of their behaviour and what they get modded for often screams "OBVIOUS GUY ... AND A COMPLETE DOUCHEBAG TO BOOT!" Things like being banned and having posts deleted for misogyny.