Well, the first way to know is to know if you were actually pregnant in the first place. Heavy bleeding and cramping signify a miscarriage.
If you didn't even know if you were pregnant, then there really is no way of knowing. It's actually quite common for women to miscarry and mistake it for an irregular period because they didn't know they were pregnant.
how far along are you? as long as you are following your doctors orders you should be fine however if it is something you are worried about the doctor will be able to tell you.
Not sure exactly but I would go to you doctor and ask them.