“How do they tell?”
They can tell because they PUT THEM THERE.
They can tell because they PUT THEM THERE.
So if I understand you correctly every single Capitol police officer was given a list of all the Antifa disruptors (and there were a lot of them, judging by the photos I've seen) and told not to charge them with anything? Yet not a single one blabbed about this scheme on social media or turned the list over to Trump's people. I find that hard to believe, given that 50% of cops are NOT criminals according to your estimate and the number of dumb criminals posting incriminating evidence on social media for "likes".
If there's a silver lining to social media turning us into a society of attention whores, it's easier to discover a crime and the more people are involved in the crime the higher the probability of leaks is.