Check backswing plane from a face-on video
From a face-on camera view you can still check your backswing plane by how much of your lead arm you see. If the club swings too much inside and around you, the lead arm tends to disappear from view; if you stay a little more over it and down the line of your feet, you see more of that lead arm and the front of it is more visible.
Channel: PorzakGolf
Watch official source at 1:06