Hedgies at a healthy weight typically either have straight sides due to being a huge runner & fast metabolism (so they often need higher fat food to stay at a healthy weight), or slightly rounded in a teardrop shape, like Abbys said. Overweight hedgies will look rounder, more ball-shaped than teardrop. Other signs of being overweight include a double chin, rolls of fat under the legs (in the armpit area), a hump of fat between the shoulder blades on their back, and they may have a "squishy" feel to their sides, which is the extra layer of fat. If your hedgie is fat enough that they can't roll into a ball at all, they're pretty much at the point of obesity, not just overweight.
I agree with Abbys, a picture or two would help if you're still unsure about whether she's overweight. A picture from the side and one from directly above her, pointing down, when she's sitting normally (not stretched out, etc.) would be the most helpful.
~*~*~ Kelsey ~*~*~
RIP my sweet Lily ~ 6/12/08 - 1/20/12