Fuel Gauge Doesnt Work

Ever since i baught my jeep in Feb. the fuel gauge never worked. Didnt bother me very much but now its just irritating. Im just trying to narrow it down to either the guage its self or if i need a new fuel sending unit. Anyone have any good tests to figure this out. Its an 89 with the 4.0. 18 gal tank. The needle on the gauge stays pegged way past full. Every once in a blue moon it will start to work but it wont read right. Any help would be much appreciated. Just incase i get grilled on the question. I did try the search button and didnt find much at all

If the sending unit isn't connected well enough (or is toast), it will read past full all the time. When I changed my fuel filter, I unplugged the pump connector (which is also the sending unit connector) and my guage pegged out at max....