Most antifascist groups are not protest groups. Most of them are watchdogs. When a story about military groups (including police) making nazi salutes or getting nazi tattoos or praying in front a a picture of Hitler come out, 99% of the time it's an antifa group that contacted a journalist and provided them picture, dates and specifics about the story. If you want to arrest all antifascists, you will have to arrest half of the OSINT community, because they're the one who developed a lot of the tools.