How do you know you had a miscarriage when you dont know you are pregnant?

It depends when you think you miscarried. If it was before or soon after your period was due, you probably can't know. But if it was a week or more after, you can do a blood test or even a home pregnancy test and if it comes out positive, you either are or were pregnant and miscarried. If you were pregnant, you would have a trace of HCG (the hormone your body makes when you are pg) still in you blood, although the level would be quickly dropping.