CleanHouse

I bought a used burner a few weeks ago but did couldn't test it then. Hooked it up today and the burner would not ignite. The only way I could get it to fire was to connect the red wire from the flow switch to the thermostat the switch has red/black/white. When I did this the burner would fire but would not shut off unless I manually turned it off. I have a feeling the flow switch is dead but I am not sure how to test it. Any suggestions? I am clueless