Remove the wire coming out of the top of the sending unit and touch it to a known good ground (it will not spark, keep your pants on) The gauge should read way full. If not it's the wires or the sending unit. The wires can be tested pretty simply with a mulitimeter, so can the sending unit. Most of these things die when the little coil in the sending unit has been rubbed by the float arm one to many times as has broken. Sometimes the float will stick, but this isn't a problem that happens that much anymore. Positive action when testing for ohms, doesn't necessarily mean the sender is good, they can work intermittently, because of gunk, multiple breaks or dents (wear spots) in the coils that make momentary contact when the float arm move past.
If you understand how to test wires with a mulitmeter, then you can test the sender too, but if testing the wires scares you off, then let someone else test the sender (it ain't hard and nothing is going to shock you or make sparks)
9 times out of ten it's the sender and it's not to tough to change out, though you will have to do some work in less then easy access spaces.