If you have a heater in a tank, and way of measuring temps other than your finger... yikes. A whole lot can go wrong there.
That's a car without a gas gauge and multiple drivers.
I have a hand held infra-red gauge I just walk along and check with. I compare to a couple of quality (but very old) battery operated digital gauges. At one point, I had the ones @WhistlingBadger
suggests. With a multi-tank set up, I bought 20 directly from China, for under a buck each. They were the exact ones stores here had for $9.95. A large percentage of them were accurate, but one in five was an outlier by several degrees. In time, I replaced their batteries a couple of times, but they didn't last.