Is it the sending unit or the gauge??

[fastlane]

Water temp sending unit doesn't work, or the gauge doesn't work. the gauge doesn't move at all, stuck in the same position since I put it in (swapped to gauges from lights) don't know if the sender works either. My research tells me I should ground out the wire from sending unit and the gauge will peg out, or see how many ohms the sender has with my absent ohmmeter!! Am I right? or is there an easier way? do you have to take the sender out to test it or can you do it in the truck?