I don't think there is a tell tale sign per se, experience tends to tell people what it is for the most part. But a little back ground can help.
What are you feeding her, how often and how much?
Water change schedule/%?
Females will become eggy once in a while, and usually within a couple of days will either absorb them or drop and eat them. Their eggs are kept right behind the belly.. so you will want to look for more of a darker bulge there. A bulge more towards the front would indicate more of stomach problem - too much food, low quality food, internal parasites, etc.. That is the closest way to try to explain, and even then it's not always accurate as bulging towards the back could still mean intestinal problems, etc.
Fast her for 3 days and see how she does then - she should go back down to normal from lack of food or absorbing.
Pictures would help, as Callistra mentioned.