It is very easy to spot someone with poor posture. But being able to feel poor posture is a completely different thing. We cannot look at ourselves, knowing if we have poor posture ourselves is difficult. The feeling of having poor posture could be so ingrained in each of us that we may have no idea that we have it.
If you find that you commonly have muscle pain, especially in your lower back or neck, then you may need to consider if your posture needs looking at.
Take a couple of pictures of yourself from the side and front. Your ears, shoulders, hips and ankles should all line up. If this is not the case then you may need to work on your posture.
As poor posture is usually caused by underdeveloped muscles, it is worth working on your core muscular development.