Conceiving after miscarriage without cycle

Ra
How can you tell when you conceive after trying again for a baby after a miscarriage?
I lost my little one earlier this month, and had bleeding for about 3wks. I finally stopped and my boyfriend and I had sex recently. We had wanted to wait for a first cycle, but also were very eager to try again.
The only issue is if we were successful, how would the doctors be able to tell how far along I am without my period to base it off of?