To referee the scrum properly, you have to be part of the scrum. I wish all referees take some time and do simulations where they are binding as a loosehead, tighthead prop and the hooker, and gets to understand what it feels like to be up front and how tough it is to scrum. Some referees guess half the time, and on other occasions, they are just wrong with their observations.
But prior to engagement the packs must be straight and still.
At engagement there must be a clear bind on your opponent, and that bind must be kept.
Prior to the ball being thrown in, the scrum must remain straight and there shouldn't be any movement by either pack.
When the ball is thrown in, there must be a clear hook by the attacking team's hooker.
While the ball is in the scrum, teams may try to overpower their opposition by driving forward and straight. the attacking team may also wheel the scrum to their advantage to give their backline more front foot space.