How do I learn to see what's wrong with someone's movement?

Right now a body in motion is a blur to you, and you cannot yet tell good from off. That is simply where the beginning looks like. No one is born reading movement. It is a language, and you are still learning its first words. The fact that you want to see it is what will teach you. Start by learning what healthy movement looks like, deeply, before chasing what is wrong. When you know the sound of a clean note, the wrong ones stand out on their own. Watch good movers closely. Learn the shape of a proper squat, a normal gait, a balanced posture. The flaws reveal themselves against that baseline. This week, pick one basic movement and study it until you truly know how it should look and feel. Then you will start noticing when it does not. You are not blind to this forever. You are simply at the start of training an eye that gets sharper every time you use it.