How to Test for Ground
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 one of the probes of your tester and slide it into the larger slot on the outlet, then insert the tip of the other probe into the small slot. Since you're dealing with AC voltage, it doesn't really matter which of the color-coded probe (red or black) goes into which slot or opening. If there is voltage in the circuit, it will be indicated on the tester. If the tester reads no voltage, then either the circuit breaker is tripped or turned off (or the fuse blown) or else, the tester could be faulty. Check another receptacle (that you know is working) for voltage to rule out a bad voltage tester. But before dismissing a "digital" meter though, it would be a good idea to make sure it has good batteries and that it is set to the proper setting. If the tester is working, reset the circuit breaker (or replace the fuse, whichever the case may be) that powers the initial outlet tested.
Testing for Ground
If you get a voltage reading between 110 and 120 volts when the two probes are in their respective slots, remove the probe from the larger slot and slide it into the U-shaped ground opening. You will know that the receptacle is properly grounded if your voltage reading is the same now as it was when the probe was in the larger slot.
If the tester does not read proper voltage when you have one probe in the smaller slot and the other probe in the ground opening, then keep the probe that is inside the ground opening in place and move the other probe over to the large (neutral) slot. If you get a voltage reading with one probe in the ground opening and the other probe in the larger neutral slot, then this is a case of "reverse polarity," or in other words, the receptacle's wiring is reversed (with the hot wire connected to the neutral side of the receptacle and the neutral wire connected to the hot side of the receptacle), thus causing a serious hazard and should be corrected.
If the tester reads 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 turned OFF until it is repaired to ensure that safety is not at risk.