The voltage reading on a multitester or multimeter tells you if power is reaching an outlet. Properly used, your multimeter also can tell you whether the white and black wires are reversed, whether the receptacle is properly grounded, and which cable entering the box feeds power to the outlet. (If you don't have a multimeter, you can use a voltage detector in place of the voltmeter and a continuity tester in place of the ohmmeter.)
If a problem arises, usually you can diagnose it by running only one or two of these tests. You conduct most of these tests with the power on, so work carefully. To be safe, hold both meter probes in the same hand so that a shock doesn't pass through your body.
A properly grounded outlet registers voltage when one probe of a voltage detector is inserted into the small outlet slot and the other probe is placed on the receptacle's center screw. If the light fails to light, the outlet is not properly grounded. Conduct a polarity test.
Test the outlet by turning off the power and disconnecting the outlet from the wiring. Set your multimeter to ohms and put a probe into one of the outlet slots and the other probe on the nearest terminal screw. The meter should indicate continuity. Test the remaining slot and terminal. Test the ground slot to the grounding terminal.
When two cables enter a box, one leads to the breaker or fuse box; the other carries power to other devices on the circuit. To determine which is the hot cable, turn off the power, disconnect the outlet, and place caps on all the wires except one black one. Turn the power back on, and touch a probe to the ground wire or the box and the other probe to the black wire. If you get a reading, it is the hot wire. If not, it is the wire leading to the other devices. To double-check, turn off the power, move the cap from one black wire to the other, turn the power back on, and test the uncapped wire.
Insert one probe into the large slot and the other against the screw (scrape off any paint to insure a good contact). If the voltage detector lights, the hot and neutral wires are reversed. If the light doesn't light with the probe placed in either slot, the wiring should be checked further.