Sexing live bearers
The way to sex live bearers including platy sp.
Look at the anal fin, the female form is a more regular "fin" shape. The fin on the male will appear to have less rays and be more compressed across its length, it is also called a gonopodium.
Edit- looking at the picture I would suggest intestinal worms, pregnant female show a darker spot behind the silver sack containing the organs, above the anal vent, as opposed to a forward area close to the pectoral fins.
4tanks2day YouTube freshwater tropical fish (stills) video, time 1:58 into the video for clear evidence, male appears shortly after.