the rook

How do I test if my temperature gauge is malfunctioning on a 91 hatch? After driving home in the evenings for an hour and then turning the car off and turning it back on the gage begins to rise and fall erratically at or above the halfway point and pushing all the way to redline. However when I open the hood even with the temperature very high on the gauge I noticed that there are absolutely no signs of overheating on the engine itself. I have taken it in the shop twice and both times they have run extensive tests and come back with nothing. The idea was brought up to me today that possibly it is just the gauge. I would like to know if there is a relatively easy way to test and see if the gauge is malfunctioning. Thanks