Just ask. I hate it when people do it without asking. It's just about manners and respect imo.
The reason I ask is that is that when I was in Germany I took some pictures of some cops arresting a 10 foot tall and bulletproof tourist, because, well, I'm a cop myself, and was completely shitfaced. Some civilians nearby began gesturing/speaking at me and talking about pictures and photographs. They were speaking German so I didn't really know what they said aside from that. Happened another time during that trip also. Third time I asked, though. I wasn't intrusive. Keep a good distance away. It's just here, if you are in public, you are fair game, no matter who you are. I didn't think about that before hand while there.
Back on topic. Hell's Angels in Europe, are they involved mainly just in prostitution?