There are different methods that we could use to assess how you stand or move. This could depend on what you are presenting. Say for instance that you wanted to improve your posture, maybe a little round shouldered not quite standing upright. The obvious thing to do would probably be try to stand up straight and hold your shoulders back. This was something that had been used overtime to try to correct the issue as it was thought that we should just be able to do it. From an observational point of view this was all that was required but could cause you further issues.
This may then lead us on to thinking about a syndrome termed upper cross syndrome, where certain muscles were pulling tight and other muscles weak or over active. Where your upper traps and levator scapular muscles become over active and your chest muscles become tight and short, pulling shoulders forward as part of the problem. Giving exercises to re establish good muscle function should restore the problem.
It could be that you have lost a little mobility in your spine, due to tightness in surrounding muscles. Perhaps you have worked in an environment conducive to being in this forward position and it seems to have become part of your position. So it may be that you just need some mobility exercises and a different working position.
What about how you breathe - are you a mouth breather? this could possibly show up as a head forward position affecting neck and upper back. We breathe around 20,000+ breath's per day and if you tend to use your shoulders and neck muscles when you breathe they may become stiff and achy. You may be better at either breathing in or breathing out that could affect how you stand. If the upper back is stiff it could affect movement of your ribs and therefore affect breathing and position.
How is your balance? Do you have to look down in front of you when you walk? Do you have to hold yourself stiff or do you feel relaxed when you walk? If you have to hold yourself stiff when you walk then it could be related to your vestibular system.
Do your feet turn in or out when you walk affecting posture further up the body. Do you sway more to one side? Do you wear your shoes out in a particular way. Do new shoes help you stand better?
And you can't rule out anatomical differences, some medical conditions or conditions that may run in the family that may predispose you to the way you stand and move.
We are not built symmetrical, liver on one side of the body, 3 lung lobes one side and 2 on the other, a stomach on one side and a diaphragm that is connected differently on the right to the left. Usually we are good at standing in a fairly upright position but we could be said to be more geared to the right than the left due to the positions of our internal organs and connections.
What about pain and posture are they related? A body of evidence in the literature does support the idea that a bad posture may cause pain, but epidemiological studies do not support that pain and posture are connected.
As you can see, there can be many influences on what is regularly termed "posture". Posture is also seen as a position that you move through, which usually has a start point and an end point.
Hopefully this has given you a few different ways to reconsider posture and maybe there are still a few more theories and ideas out there to add to the complexity of our posture.