Leaning against a wall, you stand on one foot while lifting the knee of the other leg to above hip level. The foot of the standing leg can be a couple of inches away from the wall.
If you can hold this position for at least 30 seconds, your hip flexors are not weak. Should your hip flexors test weak, you need to strengthen thaem by performing the wall hold for one minute on each side before doing the hip flexor stretch.