Water temp gauge problems.

My temperature gauge has stopped working. This is annoying. I want to replace it, but I have a problem: I don't know whether it is the temperature sender or the gauge which is at fault. I am on a budget and don't want to replace any more parts than I have to. The gauge/sender is the one-wire type, with just one wire coming from the sender to the gauge. Does anyone know a test that will show me whether it is the sender or the gauge thats broken?