How do you build scanning without shouting 'check your shoulder' every pass?

Build the look into the game rather than keep reminding them verbally. One useful practice is to change something behind or beside the receiving player before the ball arrives: a defender moves, a target changes, or a teammate shows in a different place. The player needs information to make the next action. You can also praise the scan itself even if the touch afterwards is not perfect. I would reduce the shout from the side because once “check shoulder” becomes background noise, the player is responding to the coach rather than learning when information matters. Ask afterwards: “What did you see before it came?” The long-term habit is not turning the head for the sake of it. It is looking because there is useful information to collect.