If she has a lot of fat on her belly, and a mommy-pouch there, that is one sign. The yearlings of both gender are normally scrawny looking and not worth shooting anyway.
Personally I don't hunt doe's at all. Unless there is really an overabundance of deer, I prefer not to cull the doe herd. Since 90% of the bucks do not breed anyway, these are the best to cull by hunting.
Doe's produce the next generation of fawns, and half of them will be bucks, so I prefer to leave the doe's alone and let them reproduce.