Question about Progesterone Deficiency?

I recently suffered 2 miscarriages in a row, and my doc suspects a progesterone deficiency. Should I be tested for my levels while ttc, or do I have to be already pregnant? Not sure what my next step is to see if this is the issue...and I don’t want an unnecessary appointment/bill if it’s something they can’t tell until I’m pregnant anyway!

Thank you!!