I had already had a separate blood test done to show pcos. I think when they take the blood, your gp has ticked all the different things the lab should test for so if they are looking for pcos it will be more than just progesterone. I asked my gp for a print out of my results from the pcos blood test as I found it easier to sit and look at it at home afterwards so I felt I'd understood it all, if that makes sense. Mine showed elevated testosterone and an imbalance of fsh/lh which pointed to pcos, then I had to have an ultrasound to confirm the diagnosis.
As you say - better to get tested and know where you stand, esp if you have been trying for a while.
Anna x