Update: a cabinet guy in Knoxville showed me his trick for checking square and now I use it daily
I was at a shop in Knoxville helping a buddy install a run of uppers and this old cabinet guy pulled out a 3-4-5 triangle he keeps in his apron pocket. He laid it across every carcass before we hung anything and caught two boxes that were almost a quarter inch out, which would have shown up later as a gap at the crown. Does anybody else keep a dedicated square just for checking boxes or do you just trust the diagonal measurement?
That 3-4-5 triangle in the apron pocket is the real deal, I keep a small one in my pouch for the same reason. Diagonals will lie to you on a racked box, so I check square first then verify with the tape.