Watch Some Very Good Lifts!
It helps to spend a lot of time being around technically proficient lifters so that the rhythm of good technique is burned into whatever part of your brain records such patterns. Then when you see a deviation it will jump out at you. Ultimately you will have to be able to watch the entire lift and not just one bodypart or another.
I don't know why but the rhythm of real time lifting is slightly different than the perception you get from watching video. You need to get somewhere where you can watch lots of good lifting.
The coaching eye is a long term project.