You have to contain male dogs when you have a female in heat. If the pitbull mix was with the minipin while she was in heat and unattended, then there is a good chance he mated with her. She can get pregnant by two males. Some of the pups will be from one male and others from the second male.
She may have problems delivering pit bull pups, so you will want an x-ray done about 7 weeks from the mating to be sure the pups are small enough to be delivered normally. Pregnancy in dogs is difficult to determine in the early stages. Dogs have a heat cycle during which she ovulates and "ties" with a male in order to become pregnant. If you witnessed a tie, most likely she is pregnant. One sign is a vulva that does not completely shrink down--it stays soft and a bit more swollen than what is normally seen after a heat cycle. You may also begin to see a discharge that is sticky and dries quickly on the hair around the vulva. Around week four of the pregnancy you might notice a thickening and the start of weight gain and some swelling of the breasts.
Your Vet has a few different ways to determine if your dog is pregnant. They can run a blood test to confirm pregnancy based on hormonal levels. You can read about this test here:
They can also gently palpate the abdomen at about a month after breeding to feel for puppy bumps. They can perform an x-ray during the final week of pregnancy (8th week) and look for puppy skeletons. This method can give you a good idea of how many puppies your dog may have. Ultrasound may also be available at your vet and can help determine pregnancy in a dog. Once a dog reaches the 8th week of pregnancy, you can frequently feel the puppies moving around inside of mom. I find it best to have the dog on their side and feel between the rib cage and hindquarters above the breast area.
Puppies are born approximately 57 to 64 days after breeding with the average being 63 days depending on when ovulation occurred.
False pregnancy does occur in dogs and the symptoms are the same as if they were indeed pregnant. If in the fifth week following mating your dog is experiencing these symptoms, she may be pregnant and should be fed a high quality puppy food. I feed any female that has mated a high quality puppy food until it is determined they did not conceive.
I know you were looking for a sure-fire answer but it really is difficult.