I have a pregnant mouse platy,how should i prepare?

i currently have 2 mouse platies i got from petco and i have them in a 10 gal tank,no aquarium plants and a fake log with holes fish can swim through(idk why im explaining in such detail XP),i noticed that one of my platies was abnormally fat for such a small fish(and i just got it a day ago so its not a case of over-feeding,at least,on my part),how can i make sure its actually pregnant before i see small things swimmin around in my tank?(XD)and if it is actually prego than what should i buy or consider buying or what should i consider if i want to keep some/sell some?