How can you determine which wire is hot or common when all the wires in the wall are black?

If it is in a house generally black wires are hot and white wires are neutral (common). The neutral and ground wires are tied together in the main panel.

With that said if you put 1 lead of your meter (set on AC voltage) on a black wire and the other lead to ground and you get 120 vac that wire is hot. If you get no reading it could be classified as common.

This is the normal wiring practice.

This is a basic assumption without having 1st hand knowledge of the wiring system

If in doubt contact a qualified electrician