Filter minders are pretty simple spring activated devices... not likely to malfunction. Does yours show how high a reading it goes up to?
I looked at filter minders on Ebay and found one for Cummins that goes up to 25" of H20 for
$25.. Being cheap I found this
one for $15 that goes to 25 too. You do not need the electrical connection that comes with it.
Since I have some small clear plastic tubing I would fill a 6' section half full with water and attach it to the filter minder. With a loop in the tubing and the minder at the top I would lower the open end until it goes into the red. If you have 25" of difference in the water levels the minder is good.