I am guessing you have rally gauges, and the voltage reg is built into the fuel gauge.
If your temp gauge works then it is probably fine.

if it doesn't then you check for pulsing lights on a test light. or a digital volt meter would show 5 volts.

you can test your gauge by wiring 2 AA batteries together in series to get 3 volts and touch the ends to the pos and neg of the gauge to see if it moves.

typically it will be the sending unit, and you can test that as robert spells out above.