Sometimes basic things are the most difficult to describe, but here goes:
Lift your chest while standing, and keep your shoulders relaxed. Evenly position your weight at the edge of your feet. The outside should be round the little toe, and center of the heel. (kind of like a tripod). Observe how your legs and hips are positioned naturally. If your pelvis is neutral, and knees, ankles, and hips stack up in symmetry, you can proceed with nothing else left to fix.
If a shoulder or hip is lower than its opposite, you may have a residual tension or skeletal misalignment. See a chiropractor, or you will be facing this issue with lots of useless tensing. It is also likely that one leg is larger than the other, and it is nice to know that, so you now know how to solve it correctly and use foot levelers if needed.
If your knees twist in a little, causing a slight bowl form to the legs, then you need to use muscles in your thigh to get your knees and thighs facing front, and over the ankles. Also, remember where your pelvis is here.
Now keep that position and turn sideways. Turn your head, neck, and see if you have a straight line going down through your bones, from the tip of your head, through the standard curves of your hips, spine, ankles, and legs. See if anything is stretching out of line. This can be very tricky if you are already qualified. Your body should be able to line up well without much effort. If you are undergoing a little tense, shake everything out, and then put yourself again.
If your knees over-straighten and bend backward, you have hyperextended knees. It would help if you learned to hold them straight so they can support you with strength. If your knees are bent a little forward, you can relax and stretch your leg and hip muscles, and you will get creeping development.
Regardless of your level of training, you can always review this fundamental posture to see if there is any extra tension or misalignment. It affects the delicate details of the essence of your work. It also alters your risk of getting injured.
Correcting skeletal alignment and postural habits is how you know if your great work, or skill, is hiding a possible problem. The certifying factor may be the need to intensify strength in your core muscles or do better stretching and relaxing.
The easiest way to promote your classical ballet work is to fix a difficulty where it starts, rather than trying to see what is wrong in multiple awkward turns or struggling in grand allegro.
