I wouldn't say it's an infallible guide, but my own thinking goes a bit like this:
1. Doe + round-bellied = pregnant (shoot)
2. Doe + slender + small = immature (shoot). I never leave a small doe to grow into a bigger one.
3. Doe + slender + full-grown = wait and double-check for follower (thermal helps).
I always wait after shooting: if there's an unseen follower (or a buck) about, there's a good chance it will seek the doe.
FWIW, I think overlooking a follower matters less on ground with plentiful foxes, as Charlie will quickly take any unguarded muntjac fawns.