Water Temperature Gauge or Sensor/Sender Fault

Hi
I would be grateful for some advice.
I have a 1988 3.5 and the water temperature gauge is playing up. Initially the gauges needle started showing a slightly lower temperature and after a few days also started to work intermitantly. Is it more likely to be the gauge or the temperature sensor/sender at fault and is there a way of checking whether it is the gauge?
Thanks

check coolant system is completely full ,if you remove wire off sender and earth it guage will rise to highest temp

James - tested as suggested and gauge is OK, so just need to replace sender.
Thanks for the advice.