Locate a ground source. On an industrial machine or appliance, the metal chassis is grounded and makes a fine ground source. The ground source is a return point for electrical current. For a circuit to be complete, it must have a return point. This point is called "ground" because it is usually a literal connection with the earth.
Attach the negative probe from the voltmeter to the ground source. This is typically the black probe. Make sure the probe is plugged into the "-," "COM" or "LO" socket of the voltmeter to verify that it is the negative probe. There will be no reading on the voltmeter until you attach the positive probe to one of the interconnects.
Attach the positive probe to one of the interconnects. If the voltage reading is positive, then the interconnect is the positive interconnect. If the voltage reading is negative, then the interconnect is the negative interconnect. Separate it from the other interconnect by unraveling it.
Plug the positive interconnect into the positive slot of the device you wish to connect to. For example, if you are dealing with speaker wire interconnects, the positive slot will be clearly marked by a "+" sign.
Plug the negative interconnect into the negative slot of the device you wish to connect to. For example, if you are dealing with speaker wire interconnects, the negative slot will be clearly marked by a "-" sign.