How can you tell if you have no power to a switch? Use a non-contact voltage meter to check and see if you have power to the switch. You should get a reading when you move the switch up or down. Make sure you have checked the bulbs before proceeding.
Checking the Light Bulbs
Try the bulbs in another fixture that is working before you automatically assume it is the switch.
Removing a Light Switch
You should shut the power off before you remove the cover plate on the light switch. With the power off, remove the two screws that hold the cover plate on.
After that, loosen the screws that hold the switch into the electrical box. You will need to take them completely out of the box, to free up the switch. Pull the switch out of the box a few inches until the wires that are attached to the switch are exposed.
Usting a Continuity Tester
Turn the power off and take off the cover plate for the switch. Check it with the tester to make sure the power is off. Remove the screws that hold the switch into the box. Pull the switch out and examine the connections. Are the loose or corroded? Yes, then clean them and tighten them. Turn the power back on and see if this has taken care of the problem. Yes, then it is a job well done. No need for wiring a three way switch. No, then the switch is bad and needs to be replaced.
Does the meter show that you have power to the switch? Yes, then the switch could be bad and need to be replaced. Or the wiring could be loose or corroded.
Using a Voltage Tester
There are different types of voltage testers. The cheapest ones sense when current is going between two point. Two use this type, you will place the leads on each terminal of the switch. Use extreme caution when performing this test, it will only work when the power to the switch is turned on.
They have two leads and a light the lights up when current is sensed. This type will not work properly when used for a light switch. The problem is that the light in the tester acts as a bridge for the circuit, when the switch is off. It allows current to pass through the filament in the tester. Thus current will show for both an on and off condition, making you think that the switch is bad.
The only way this will work properly is if you remove the light bulbs from the fixture. With the light bulbs removed the tester will not light up when the switch is off. It should still light up when the switch is turned on.
Using a Non Contact Voltage Tester
A much better option for testing a switch is a non-contact voltage tester. This type of tester senses when there is power in a wire. You can first check to see which side of the switch has power in it. Once you have located the power source, you can toggle the switch on and off to determine whether the wire on the far side of the switch is getting power when it should.
You need to be careful when performing this test. The power needs to be on when the test is performed.