Eh, I could write quite a bit about how tools have changed, and how I have ended up buying stuff at Harbor Freight when I used to despise the place, but that is not what this is about. If you want to go old school to check the validity of your gauge, get a water dropper. You can get one at any pharmacy for about $4. Just buy an ear wax removal kit, and there is a bulb dropper in it. You can still use it to clean your ears, which by the way, I would like to mention is a life changing revelation. Drip water on the primary and visually measure how long it takes to vaporize between different primaries. If those two primaries appear to vaporize water faster than the others, then it is time to start pulling spark plugs to see if there is any difference in the appearance of them. There is a spark plug reading chart in the back of any Haynes manual. We can go from there.
Kurt