Put a male Gambusia of almost any species in with it/her and if there is no gravid spot within 30 days, you can be almost positive it's not a 'skeeter fish.
Also, try to get a in-focus close-up of the mouth. If it's not oriented upward you can cross Gambusia off the list.
It is surely a live-bearer (Poeciliidae). Since, from what I can tell the origin of the anal fin is in front of the dorsal fin. The head is a bit more elongated than what I'm used to for mosquitofish, but if I had to guess either the western or eastern mosquitofish. It is quite common for females to be missing the spot.
Im still sticking to gambusia. Young never pregnant females will nit have the black spot. And its missing the gonopodium because its female. And its not to long that what i consider normal body for gambusia. But ok.