It's easy to test the switch with a volt meter. All you have to do is test for continuity on the leads where the wires plug in on when you press the switch. You should have continuity when you press the button and no continuity when you release. Just remove the 3 screws to get the switch out and disconnect the wires and test. If the switch is good than it is on the other selector where you toggle back between constant and momentary, which also can be tested the same way.
If you are not good with a tester and the read out values just put it on beeper. Touch your leads together and it will beep, any time you have continuity it will beep. Put each line on a lead and press the button and listen for the beep its very simple. I know you already ordered the part but next time it might not be $17 you may need to diagnose first.