If so then don't bother detecting it; just provide a known shortcut that live-streamers can toggle to enable livestream features. Less hassle, more predictable
Fair enough, and I'd agree with you 99% of the time that that's the appropriate way. The particular user-experience-improvement features I'm thinking of won't work if the user has to explicitly enable them, though. (That's probably hard to believe; all I can say is my game is not a typical game and what I'm thinking of is very, very meta.)
I'll keep the objections raised here in mind. I've been watching some streams and I have few ideas on how to divine whether someone is live-streaming behaviorally. If it proves to be something I can't figure out how to do inoffensively enough, I just won't put the feature in.