I always though this was just to tell them apart, as others have said. Failing that how do you? What makes you pick up an unknown bow and say "this is a viola bow" or "this is a violin bow"? A viola bow will *tend* to be a bit heavier, a bit longer or have a different balance point - but not always in any of those respects. In most cases the maker intended the bow for one use or the other, but once it leaves them how do you know? Do you just pick up an old bow and infer that, regardless of the maker's intent, this works better as a viola bow and I will label it as that - henceforth this will be known as a viola bow.
The fact is you can't always tell whether a random bow with which you aren't familiar is a viola bow or a violin bow. Heavy violin bows can weigh the same as light viola bows. And to muddy the waters, some violists sometimes use violin bows to play the viola and some violinists sometimes use viola bows to play the violin. I can tell my viola bow with a squared heel on the frog from my violin bows with squared heel frogs because I recognize the pearl eyes and because my viola bow has a narrow strip of leather at the end of the wrapping and my violin bows don't. The sticks look different too. And, as Martin has pointed out and so have I, there are plenty of violin bows with rounded frogs.
P.S. Viola bows are generally about the same length as violin bows so no help there.