Video is a great tool for this and it really does help. When I was missing right last summer, my driller (also a certified coach) did video on me and found 3 different things. My wrist was ending up in a broken position instead of staying cupped, right shoulder was staying open too long, and my timing was getting me into the swing too early. Based on this, I don't stand 3/4ths of the way back on the approach anymore- moved closer to 1/2 way back and shortened the steps in my 5 step approach to fix that.
As for the wrist and shoulder- my driller had me do drills where my swing is slightly less high than it used to be (now shoulder high instead of slightly above) and this helped me close the shoulder quicker. Then, I just worked with keeping my wrist where I need it to be.