Test the switch for continuity using a multitester. Set the multitester to the ohms setting X1. Touch a probe to the side terminal. Touch the other probe to one of the two other terminals. The reading should be either infinity or zero. With the probes still in place, depress the switch. You should now get the opposite reading.
Keep the first probe on the side terminal and move the other probe to the second terminal. The test results should be the opposite of the first terminal. If you had a reading of zero on the first terminal, when the switch was depressed, you should get a reading of infinity on this terminal.
If the switch fails either test, it should be replaced.