Test the switch for continuity using a multitester. Set the multitester to the ohms setting X1. Touch one probe to the side terminal and the other probe to the nearer terminal. The reading should be either infinity or zero. With the probes still in place, depress the switch. You should now get the opposite reading.
Leave the probe on the side terminal and move the other probe to the farther terminal. The test results should be the opposite of the results of the nearer terminal.
If the switch fails any of the tests, it should be replaced.