In the fall, winter and early spring, males will in many cases be leaking milt, you'll see it. As mentioned, they're also smaller and won't have a bulge in the egg sac area.

Females are quite larger and when loaded with eggs there's no mistaking the egg sac even from the outside. After dropping, that area will be stretched out and equally noticeable.

Spring and summer are a bit more difficult to tell but larger specimens are more apt to be females.