Many times older males (ie ones that have been calling for some time) will have a darkened patch below the chin. I wouldn't say this happens all the time, but it certainly helps. You can also see the throat skin look like it can stretch more when regular respiration occurs to see the similar thing-- but again only with mature males who have been calling for some time. I've seen more sexual dimorphism with D. granuliferus than D. pumilio.
One thing you do NOT want to do with pumilio is judge by body size. I've seen a male so big he looked like a female who has a bazillione eggs in her. Right after a male has called his throat will turn grayish just like Yeager said, and that's a good way to sex them.
Somewhere on Patrick's site (saurian.net) he had a little article that mentioned how he sexed pumilio. It involved moving them into a new container or something... I dunno, its been a while since I read it, but it made sense to me (and works with other frogs too).