You can test an outlet to determine if current can flow with a voltage tester. Always test your test equipment for proper operation before use. If you don't have a voltage tester, simply use a shop light or other convenient electrical device. Start by making sure the tester is working and plug it into a circuit you know is working. Note that if you need to test a 220V outlet, these instructions do not cover that test.
There are a variety of testers to choose from, the most basic is pictured below. It has two probes, insert one into each slot and if voltage is present, it will light up. Be sure to test both outlets, sometimes each is wired separately or only one of the two is working. To test whether the outlet is properly grounded, follow this link to the article on grounding.
If there is no voltage, make certain that the outlet isn't controlled by a switch. Try all nearby switches and check whether the tester lights up.
If you are troubleshooting an outlet that isn't working, some possibilities include:
The outlet may be in a circuit with a GFCI outlet (ground fault circuit interrupt). If the GFCI outlet has tripped, it may cause other outlets on the same circuit to lose current. Look for an outlet that has a "Test" and "Reset" button. They are often located near water such as in a bathroom or kitchen. If the outlet has been tripped, unplug anything that may have caused the fault and then press the "Reset" button.
A wire connection has become loose. A wiring fault can occur in many places, the most common include the outlet box, another outlet or junction box the wire passes through or at the circuit breaker.