The only sure way is that a boar will never be accompanied by cubs. Other than that, just try for a big bear. I would pay much more attention to learning how to differentiate size than sex.
+1
+2 "Short" legs with belly dragging the ground plus wide head with smallish ears set down over the corners and it is a shooter, whatever the sex, most likely a pot bellied old boar. If it is "tall" with long legs, it is almost surely a young bear, though I saw one huge boar in spring that was thin that fooled me for a bit.
