A mate of mine was a player on a local Sunday league team - he was probably the best player on the league and scored a lot of goals in the top division. I saw him at work one Monday morning after he’d played against a team with a reputation for dirty tricks, and he showed me his arms. They were black and blue because they were so bruised. The team had taken it in turns to pinch him, trying to get him to react so he’d get sent off. They frequently did this sort of thing to get an advantage.
I had the return fixture, so was watching out for it. I caught the goalkeeper kicking the ball out to his right back with his left foot, and followed the track of the ball, before looking back and catching the keeper kicking the forward with his right foot. Obviously dismissal and penalty.
As you say, it’s hard to spot, but if you’re aware not impossible. If you know this is a possibility then try not to get caught up with play and occasionally look at the problem player out of the corner of your eye. If you look directly at him he won’t do it, but if he doesn’t know you’re on to him he may. If you get a position where the play is between you and him then you can scan both at the same time and may catch him.
If you see him and don’t send off then you’re risking match control, and not even trying to catch him is playing into his hands. If he always gets away with it there’s no downside for him. If he gets seen and sent off it’s a risk and he may stop. Even just obviously looking at him occasionally means he will know he may not get away with it so is less likely to do it.