The main indications for adjustment are

Rounded Back and/or very high knees.  Place one hand on your spine at the back of your waist. If you feel vertebrae poking out, then you need to elevate your sitting bones to remove the struggle and help your spine lift. Be sure to sit on a high support (a stool, bolster, firm cushion or one or two folded blankets), even as high as a foot off the floor; create enough height so that your knees are no longer higher than your hips.     When back is rounded or knee very high it is likely the hips or groins are tight.  Raising the pelvis on a blanket and help minimise the rounding of the back and provides better opportunity for an opening as well as minimises the risk of injury.   Also, you can hold onto a strap around the feet if your back or shoulders round.

If you try to come forward and the adductors are not flexible, you may not be able to bring the torso forward toward the floor: moreover you may struggle to even sit upright, rather than being able to forward bend deeply into the pose.

Also I find when I have pain on the inside knees (happens more often than I like) that I benefit practicing Supta Padangusthasana first (where the elevated leg is brought downward laterally and the other leg firmly held straight).  The benefit is that 1) the inside of the thighs are increased 2) the external rotation of the thighs extended.