If they do float when dead, they usually have some massive bloat, like the dead frog on the right, or some other yucky disease which is really obvious. The dead frog above on the right died of dropsy. Unfortunately, with the Dwarf frogs it is very very hard to tell. As a rule, the females tend to be bigger than the males. Males have a very distinct white bump there.
Some females also have a white patch there, but in males, the bump is very very distinct.
Ive also heard my male frogs make a low humming noise as a mating call. It seems to me that the easiest way to tell, and this is what tipped me off to the fact that Jake was a SHE, was to wait till it gets warm enough for, er, snuggling season. The one that clamps onto the other and piggyback rides is the male. Well, I'm not exactly sure. The longest living frog in my collection has been around since about 1996.