A couple of things you might want to look for:
1. Angle between anal and ventral fins and size of gravid spot. The steeper the angle and the larger the spot (you may be able to see the eyes of the babies in here) the more likely your fish is about to give birth.
2. Abnormal behavior. If your fish is remaining stationary/inactive either on the bottom, surface, or withing the confines of bushy plants she may be symptomatic of dropsy. Also if the fish ceases to defficate or does so abnormally she may also be symptomatic of poor health.
I have had this illness affect my several of my older female platies, but hopefully your guppy is just pregnant.
Eheim Pimp #181
29g High-tech jungle
5.5g Low-tech jungle
2.5g x 2 Low-tech shrimp and moss