I have always found the head to be a good way. Big bear, head will look small compared to the body with the ears being less and less prominent. Also big bears will have quite a crease between the ears, fat, muscle, skin folds.
Sows will look "hippy". Bigger rear end, higher than the shoulders. Boars will have bigger shoulders, less in the hips and butt. A big boar will have shoulders much wider than the head.

Just my two cents.