It is harder and harder to understand what is good posture, because there is hardly anyone that has it. 99.9% of people from children to adults have some spinal deformations.
Even our graphic depictions of humans, like cartoons or street signs, are represented by humans with bad posture. Like the stick figure from the pedestrian crossing sign, his head is not in lign with his shoulders.
Our perception of how healthy human being should look has changed, without even realising.
I would show you a picture of it, but earlier when I searched some stock photography website for one, there were no examples of it.
This makes it hard to self diagnosed yourself, because if you compare yourself with the people around you, there is probably no larger difference between you and them. You might draw the conclusion that there is nothing wrong with you, because your back looks just like everyone's else's back. So whatever symptoms you might have are not caused by your posture.
You can easily spot someone with bad posture or diagnose yourself based on a few signs.
The feet never lie, it is impossible to have misaligned feet and good posture, if your feet are not parallel with each other, it is a guarantee that your spine is not well aligned.
Penduling arms, when you look at someone's arms resting along his body from the front, you shouldn't see the back of his hands. This is a sign that the shoulders are misaligned do to bad posture.
And probably the easiest one to see the head that is not in lign with shoulders, the head should be positioned directly over the shoulder, not in front of them.