video tape them and whatever cars they arrive/leave in and publish the video.
Funny thing. France just outlawed that very thing.
Video won't matter, based on personal experience. We could have video of somebody assaulting one of our people (I did) and the DC police would do nothing (they did nothing), saying that the person videotaping (me) was too close to the protesters who repeatedly struck their target (me). I told that MPDC officer, and his superior (Lieutenant Michael Farrish) that should I ever be attacked again while I am in their jurisdiction, I will not depend on them as they always insist we do, but will defend myself using all means at my disposal.
I have zero faith in the DC police doing anything to stop or to arrest any vandals or other violent peace rioters. My faith in the US Park police is only slightly higher. Maybe they will prove me wrong.
That said, we should still take photos and video on the off-chance that someone in police leadership might have the testicular fortitude to do their job.