I have had 3 platies die of dropsy...(because I overfed...but thats another story)....
In all 3 cases, it started as what looked like a graze on the side of their body...the next day the scales appeared to be raised....by that night, the whole body had raised scales....the poor fish looked like an orange pine cone

not knowing what it was, I didn't euthanise, which I wish I had as the next day the poor platy was so puffed up it couldn't close it mouth...obviously it couldn't eat. The whole time it was in the pine cone state it stayed in the one spot...when it tried to move for food it was like it was too hard for the poor little thing. Weird part was that when I FINALLY read about dropsy, I decided to euthanase it....(was crying as it was the first time)....I went to the tank and she had died in the time I had last looked at her to when I had decided to "help" her.
It is clearly different from a pregnant platy which is only big around the abdomen (like they are gonna burst the skin)