Helmet mirrors are useful for this stuff. No need to guess, or turn and look. Whether your goal is to torment your would-be overtakers by putting on a nice 30-second "turbo boost" juuuuuust as they're about to close up with you (one of my favorite forms of torment, on-road or off-road), or to orchestrate an opportunity for them to pass in a good place, or to watch for a nice gap in traffic to use for passing, being able to see backwards is helpful.