Originally posted by BillJC
You can check the voltage but, it would probably read normal
It won't if the problem is too much resistance in the circuit. Not only that, but it will tell you whether the problem is on the ground side or hot side of the circuit. Plus, it tests the whole circuit all the way back to the battery. Another bonus is that you don't have to disconnect the battery.
voltage lower than battery (+) on the hot side or voltage greater than battery (-) on the ground side is directly related to the resistance of the circuit.