Notice in the picture above, the platy has a squared off abdomen. That is a clear sign she will give birth within a a day or two, though not all platy's will square off. Every fish is different. Stress can cause them to birth before they are ready.
My platy looks almost exactly like that. Shes not completely squared off in the front but she almost is and other than that her stomach looks exactly like that. Maybe 2 days I think?
Is that pic of platy not pregnant a joke or do they just look like that