They check for progesterone at 7 days past ovulation.
Mine has indicated ovulation, but is lower than it should be even for a natural cycle and I was on Clomid.
Its possible that if I am pregnant, my body could totally kick in and up the progesterone production and everything would be great. But its also possible that my body wont' keep making the progesterone and I could lose a pregnancy.
They usually diagnose it during a pregnancy by blood tests if they suspect something is going wrong.
Otherwise, they suspect it might be a contributing factor to early miscarriages, but they don't even suspect it until you've had 3!
Its kind of hard to tell if progesterone levels are falling because the pregnancy is failing or if the progesterone falling ends the pregnancy.