Find the field wire - the smaller wire going to the alternator. The regulator controls the alternator through this wire - it incerases the current flow to this wire to make the alternator charge more. Hook a multimeter up so you can check the output (like across the battery terminals) then jump current to this wire momentarily. Just for a few seconds - its probably a little hard on the alternator to go full throttle like that. Check the multimeter to see if voltage goes up when you do. If it does it means the regulator isn't properly signaling the alternator to charge (wiring or regulator bad). If this doesn't produce any rise in voltage (or very little) it means the alternator isn't working.