Check the source code. Looks for "rss". If that returns too many hits then search for "application/rss+xml".

That's actually what I've been doing, but sites that very clearly should have an RSS feed (specifically, our local governments' event calendar pages), don't, so I thought there might be some other route/heuristic/whatever that I've been missing :-(.

Exactly the approach that I've been using for years. Manual, but works!