Agree with Montague. Mirrors are better suited for BB'ing, if at all. Helps, with moderate weight & form, to establish a better connection to the vision of you doing the exercise. Been to a garage gym where the owner had a overhead mirror above the bench rack, which kind of threw off the whole exercise for me. Also had a 3 way mirror, like in department stores. That worked kind of well.
Best to have someone who knows the lifting game observe your lifts. The best option would be a video , both side and rear views. Good Luck.