One way to check is to watch it during warm up. The T stat is fairly accurate... the guage will climb then level out, for at least a while, as the T stat opens. With a 180 stat, it will start leveling out around 185 degrees for example.

On my stock guage, 180 is the lowest end of the operating range on the stock guage... not up to the middle. They can vary a lot from car to car.