How do you check 480 volts line coming from comed?

This high a voltage is an industrial voltage. If you are concerned about variations in the voltage call your utility supplier to get them to check it for you. They can connect a recording volt meter to the line an it gives a print out over a 24 hour period. If that is not an option call a qualified electrical contractor and ask him to take a reading for you. A voltage of 480 volts is not a voltage that the ordinary layman should be opening panels and

getting into.