This indicates that electrical continuity exists through the switch. If the specific breaker is not identified turn off the main power switch.

The Best Non Contact Voltage Tester Reviews By Wirecutter
How to test a light switch without a tester. Youll need screwdrivers usually one flat head and one phillips for removing the switch and disconnecting the switch wires. Pull the light switch from the wall. Testing any type of light switch takes a few basic tools. Touch the probes to the copper terminals on the side of the switch. Testing a switch with a voltage tester. According to the university of washington you should see a reading of less than 25 ohms.
Toggle the switch into the on and off position with the test leads attached to the terminals of the switch. Remove the screws holding the cover plate on the light switch with a slotted screwdriver. You also need a non contact voltage tester to check the switch and wires for power before touching them. This video will show you how to check your light switch and how to replace a light switch step by step. How to tell if an electric light switch is bad. Remove the switch cover plate by unscrewing its two small screws to test whether it is safe to proceed to replace or rewire a wall switch.
How to troubleshoot light switches and replace it can be easy and diy project. If the switch is good then there will be a reading or indication otherwise no reading or indication means that the switch is bad. Your switch may be attached to a frame by screws which youll need to take out or it may just be sitting up against the wall. Once youve confirmed that theres no electrical current in your switch you are ready to remove it. Turn off the power to the light circuit at the breaker panel. You will need to take them completely out of the box to free up the 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. Pull the switch out of the box a few inches until the wires that are attached to the switch are exposed. Turn the switch off and test again.