You can't be sure? Are you sure?
Voltmeter at the battery terminals while running will tell you your operating voltage.
IR temp gun scanned around the thermostat area when fully warmed up will show your coolant temperature.
Re-position needles to read correctly based on measured results. Super easy to do with the lens popped off the cluster but the cluster installed so you can operate it.
Of course same goes for oil pressure (mechanical gauge), speedometer (compare to GPS), and tachmometer (attach external diagnostic tach to ignition)...about the trickiest one to do is the fuel gauge since we don't literally know the level inside the tank, can only read resistance from the sender and hope it's right (which we know it often isn't.) When I had to reposition needles on a cluster, I did the fuel gauge by filling the tank full, setting the needle to "full", drove until it reached the halfway mark then filled up again noting how many gallons it took to fill, compared against tank capacity. It was correct.
Richard