There are two kinds of testers you should have in your electrical tool box for use around the house. First is a simple neon tester. This simple device has two probes and a neon bulb.
Neon tester - detects voltage
One probe goes on the neutral side and the other goes on the hot side of an outlet. For a light switch put one probe on ground and the other probe on the black or red wire. If the neon glows or lights, there is current and you have NOT turned off the current to this device.
The neon voltage tester will detect any voltage above zero to 500 volts. It's job is only to let you know if there is current present or not.
Until you test, approach all outlets and switches as if they are "hot" even if you think the breaker is off.
For outlets, you should use a different kind of tester that provides more information. Even if you choose to use the neon tester, it simple shows if current is present or not.
Polarity tester - checks ground, neutral, hot connections
A polarity tester also shows if the ground is working and if the hot and neutral wires are connected correctly. This tool should be part of your "finish" to ensure outlets are done correctly.
