The quick, easy test of alternator output is to check the voltage at the battery
terminals. Assuming you have a basic volt meter just set to DC Volts and put the red prob on the + of the battery
and the black probe on the -. Test first with the engine
off and look carefully at the voltage. Then have someone crank the engine
while you watch the change in voltage. CAUTION!!! Make sure you the meter and wires are all clear of any belts, pulleys, etc while the engine cranks and runs.
If the voltage goes higher than 13.5-14.0 V then the voltage from the alternator is too high.
You can also check for spikes or surges in the output. This is easier if you have an old fashioned needle type meter instead of digital. Watch the needle or digital readout to see if they bounce up and down a lot.
If things look good at the battery try in the cabin
at the power distribution pannel or light sockets in the fixtures where the bulbs are going out. Be careful not to short out a hot to a ground if working in a spot with lots of wires close together.
Testing amps is a little trickier if you have no experience with meters.