To confirm luteal phase defect you should have the serum progesterone level estimated on day 21 of a 28 day cycle, or after 7 days after ovulation has been documented by ultrasound (in those not having regular 28 day cycle). Also, it should be done at least in 3 consecutive cycles to be sure. In older times, an endometrial biopsy was done premenstrually to check whether or not the endometrium was out of phase. As it is an invasive test and is not done these days.