Answer to: Tell me how to indentify a moose as bull, cow or calf?
See the cow moose vulva patch below her tail
You are quite right that Bull moose do lose their antlers in the winter, but most if not all hunting jurisdictions hunting seasons are long before the moose loose antlers. It is safe to say that during most moose hunting seasons all bull moose will still be wearing their antlers. If you only have a Bull moose tag, don't shoot at any moose that do not have antlers.
Be sure to correctly identify your target before shooting... remember safety first.
If your moose tag requires you to harvest moose with only certain size antlers, study them well before you shoot.
In photo 1 you can see the cow moose vulva patch below her tail.
In photo 2 you can see the large size difference between cow and calf... note how short the calves face is compared to mother.
In photo 3 you can clearly see a bull urinating.
In photo 4 you can see the results of a freshly lost antler
Good luck with your hunt.