Originally Posted by Mxfrank

A vacuum switchover valve works exactly like an electrical switchover relay. The power connections control the electromagnet winding, on or off. There are three vacuum connections. One connects to the vacuum source. The valve will connect the vacuum source to one or the other of the outlet ports (never both), depending on whether the winding is powered.

All you need to test is a power source, and a vacuum source. A small 12V battery and a mityvac are fine. First connect the mityvac to the source nipple. Then pump it and make sure that it transmits vacuum to only one of the outlet nipples. Finally, attach your battery, probably a good idea to mind polarity. Then check that the mightyac now connects only to the other nipple, in other words, vacuum has "switched over". Simple.