How does a doctor know you had a miscarriage if you did not even know you were pregnant?

Because of the levels of a certain hormone in your body and the way the lining of your uterus is. There are lots of ways for a doctor to know if you had a miscarriage.

He can give you a blood test right afterward that will test your levels of HCG in the blood. If it is more than two, then you were pregnant.