At that stage the best way to tell is to have the vet palpitate her and or ultra sounded, to find out. If you have the stallion still you can tease her with him but that's not always reliable some mares are just plain hussies and will wink, and pee or will allow a stallion to cover them even when they are pregnant. Good luck either way with your mare.
As someone else said many mares will still act up around a stallion even when pregnant
You can get online kits so you really need to do that or get your Vet out.
I hope the stallion has now been removed from contact with the mares or has been gelded so he can lead a normal life