Even if you don't know much about cars, you know your own car. That knowledge can help you explain a problem to a professional.

If your wiring physically checks out fine, then you've just narrowed down your gauge problem to either the sending unit, the gauge itself, or the instrument voltage regulator (if equipped).

Diagnose electrical problems by chasing voltage drops, instead of the old-fashioned way -- checking continuity. Including Mike Allen's Physics 101 introduction to Ohm's law.