duck:
Be sure to watch for cubs first like car said. After that it's anyone's guess on medium-sized bear, again like car said (car says everything

). Although, if you have the time, you can wait until the bear has a full stomach, climb down from your stand, roll it over and look! LOL! Of course, then you'll have to roll the bear over again, climb back up to your stand, get yourself ready for the shot etc., etc., etc., all without the bear noticing or catching on to your intentions! That procedure tends to be somewhat difficult and problematic, and is something I would not advise doing.
On the flip side, you'll have no problem identifying a big boar. They just have a way about them. The way they walk and act just screams, "Big Boar!"
Anyway, there's nothing wrong with shooting a big, old sow that is either dry or without cubs. I once had an encounter with a HUGE blond-phase sow in Alberta a few years back that I wish to this very day that I could have shot, but she was the doting mother of three fuzzball-sized, multi-colored cubs. She was beautiful, but brother was she surly pig! She would growl and pop her teeth and smash small saplings in her destructive path just to let everything and everyone around her know that she didn't like anyone or anything getting too close to her babies. At one point, she ran straight at my stand tree, stood up on her hinds legs and slapped the tree! I thought I was going to have to shoot her after all, but she finally backed down and walked herself and her three little ones off into the Alberta wilderness never to be seen again. Talk about the "pucker factor". Whewwwwww!
Good luck!
Take care and Good Hunting!
Best Afield,
Steve