Actually, female bettas do tend to have a very swollen belly area if they are 'gravid'... or full of eggs. Here's some photo references to help you.
Normal female betta:http://z.about.com/d/freshaquarium/1/0/R/S/betta04G.jpg
http://upload.wikimedia.org/wikiped...ta_closeup.jpg/800px-Female_betta_closeup.jpg
Note that there is no swell to their bellies, but it's not sunken in from starvation, either. A average and healthy look for a young female or a female too old to produce eggs.
Well-fed female betta: http://z.about.com/d/freshaquarium/1/0/S/S/betta05G.jpg
http://www.bettatalk.com/images/F_opaque_female.gif
These girls are well-fed and are either being conditioned through good feeding to produce eggs or were photographed just after a heavy meal. See the slight belly-swell?
Gravid female betta: http://www.bettatalk.com/images/too_much_food.gif
http://www.wetwebmedia.com/MiscFishPIX/betta1.jpg
http://www.petfish.net/articles/pics/betta_egg_tube.jpg <== Upper Right
These females are bursting at the seams with eggs. This is normal for the first couple of years in a female betta's life. If she hasn't bred within 2 years, she usually stops producing these big round bellyfuls of eggs and is no longer suitable to breed because she will no longer desire to. Females don't need to breed to be 'relieved' of their bellies. They will reabsorb the eggs as they become too old to fertilize, so she will go through fat and thinner phases as she does this.
Female betta with dropsy:http://www.nippyfish.net/flashdropsy.jpg
http://www.bettatalk.com/images/dropsy_copy.gif
http://www.waynesthisandthat.com/images/dropsy2.jpg
http://www.aquariumcorner.com/images/dropsy pic2-photo contributed by Tom Byrne.JPG
The last image was a male betta, but you can see how easy it is from the previous photos to confuse a gravid or full-bellied female with a sickly, bloated, or possibly dropsied female. The best clue is to look at them from a top-side view. In the 'dropsy pics' before the last two, you could clearly see the scales standing up when seen from above. If the female is merely full of food, fat, or eggs she will have normal, smooth-scaled sides.
Hope this helps!!