Yep a meter is the way to go. Place to start is the batt then the field input then alt out put .
Batt volt is what it is 12v to 13 v ig on car not running important doors closed and all stereo, electric stuff off ect then ck the field wire
Field =small red wire going in to the alt should be the same as or close to batt volts give or take .5 volts car not running
If all is ok here then start the car and ck voltage out the big red wire should read 13v to 14v give or take .5 of a volt if all is ok . Post what you found and we can go from there .
Maybe Spenc might want to jump in here as he is the expert .
Len