Did you only get females?
How long have you had them?
Any other livebearers in the tank, like swordtails?
Yes, there's a decent chance one or all of them have been exposed to a male at the fish farm or in the store (if males are in a tank next to females, they can easily jump into the girl's tank and have the time of their lives). There's also the fact that all molly fry look like females for the first few months, so if the breeder didn't separate out the males as they matured and developed quickly enough, the young males may have already mated the females.
Because you're correct that female livebearers like mollies store sperm packets, so can produce fry for months, even up to a year or more, even without a male around if they've had a male around them at any point, and it's common for people to buy a female livebearer then be surprised when fry begin appearing even without a male in their own tank.
But it sounds as though you're aware of that and prepared.
If you can post some decent close up photos of your fish,
@emeraldking is a livebearer expert, might be able to spot if any are gravid if they're far enough along. But they can also delay releasing a sperm packet until they're ready to risk becoming gravid, so if you've only just got them, or they're in very early stages, it can be hard to tell, and it'll be a "wait and see" kind of situation.