if you are talking about the contacts you would never read the voltage drop as it will be uV probably.
There you go. That means it's working if the two samples match. If one is 12Vdc and the 0V then there's a problem.
If both are high or both are low then your good to go. This is the Exclusive -OR function. If both are high or both are low, output is low. If EITHER input differs from the other , output is high. 12V CMOS 4030 Ic.
http://www.cmos4000.com/functions/exclusive-or.html