How to Test for Ground
Testing for ground can be performed using either a multitester or a basic voltage tester. This test ensures that the ground on the circuit is connected to the outlet and that it is working.
If you are using a multitester, set the tester to read voltage (V). If you are using a basic voltage tester, then you do not need to do anything to the tester as testing for voltage is its only function.
Testing for Voltage on a Three-Pronged Outlet
Three pronged outlets feature two slots (one large and one small) and a “U” shaped hole. The small slot is the “hot” side of the receptacle and the large slot is the “neutral” side. The U-shaped hole is for the ground prong.
Take the tip of the black probe on your tester and slide it into the larger slot, and the tip of the red probe and slide it into the small slot. If there is voltage in the circuit, the tester will indicate it. If the tester reads no voltage, then either the circuit is off and you have to turn it on or the tester could be faulty. Check another receptacle (that you know is working) for voltage to rule out a bad voltage tester. If the tester is a digital model, make sure it has a fresh battery and that it is set to the proper setting.
Testing for Ground
If you get a positive voltage reading when the two probes are in the two slots, remove the black probe from the larger slot and slide it into the U-shaped ground hole. You will know that the receptacle is properly grounded if your voltage reading is the same now as it was when the black probe was in the larger slot.
If the tester does not read voltage when you have the red probe in the short slot and the black probe in the ground slot, then keep the black probe in place and move the red probe over to the larger one. If you get a positive voltage reading with the probes in this position, then this is a case of "reverse polarity," or in other words, the receptacle was wired in reverse (with the hot wires connected to the neutral side of the receptacle and the neutral wires connected to the hot side of the receptacle) and it should be corrected.
If the tester reads positive voltage when the two probes are in the top two slots but it does not read voltage through either slot when you place the probe in the ground hole, then the receptacle is not grounded. You should check all of the remaining receptacles in the vicinity to find out whether this is an isolated incident or there is a larger concern. In either case, a non-grounded receptacle should be repaired as soon as possible to ensure it is safe for use.