take videos of yourself, and get them analysed. You can do that here or on some of the threads on the /r/fitness subreddit. The best option would be to pay for a coach (either one session or one over time) either in person or remote, but that is of course money.
Gorilla grip tripod thing for phone an record yourself from a few different angles and over a few different weights.
In front of a mirror is useful too for instant feedback
What's the best way to check and dial-in form for deadlifts?
I'm pretty sure that mine is shit.