Should the Senior Deacon look around to make sure everyone is on the Sign of Fidelity before attending at the Altar?

No.

Oh, you want more explanation? Sure. He doesn’t do this because it is not, nor has it ever been, a duty of the Senior Deacon to ensure that all present comply with our forms and ceremonies. This duty resides with the Worshipful Master alone.

If a Brother is not at the Sign of Fidelity when they are supposed to be at the Sign of Fidelity, the Master can choose to remind him or choose not to remind him. It’s the Master’s decision. Keep in mind that if you, as the Master, choose to address the Brother who is not at the Sign of Fidelity, you risk breaking the magic of the ceremony. It is wise to remind our Brothers of proper Lodge ritual and etiquette but it is best to keep any constructive criticism until after the Lodge is closed and in private.