Internal rotation is the amount that your femur (thigh bone) can rotate inwards with respect to your pelvis. To determine whether or not you have adequate internal rotation, have someone measure your hip angle in a seated position when you actively rotate the foot outwards.
You can have this done with a goniometer, which you should be able to find in any physical therapy clinic and any physical therapist can quickly measure this for you.
If you do not achieve 30 degrees doing this motion actively, have it done passively. This means having the PT or the person measuring the angle move your leg to the end of your available range of motion. If you still do not achieve 30 degrees on both hips, this is a good stretch for you.
For the prone internal rotation hip stretch, lay on your stomach with one knee bent to 90 degrees. Let the foot fall out to the side and gently press with the hand on the same side so that you feel a stretch within the hip. Hold 30-60 seconds.
If you were able to achieve 30 degrees passively but not actively, you may have a stability issue. How to address this will be addressed next week.