I use my wife's candy thermometer stuck in the radiator, with the cap off of course. I just let them idle up to temp and see if the gauge reads the same as the thermometer.
I cannot overstate how much I agree with RRR. I have an IR thermometer that is about 15 years old. I paid about $100 for it back then. For only about $20, everybody should have one in their toolbox. I use it ALL the time. I bought it for powder coating, but it comes in really handy. You can use it to double check a temp gauge by aiming it at the thermostat housing. If the engine is missing, you can aim it at the header tube or exhaust manifold port for each cylinder. If 7 cylinders ate around 400 degrees and one is around 300, that is a good place to start. Plus it is fun to do things like measuring the temp of the driveway or a car dash on a hot summer day. LOL