How do you tell if a voltage regulator is not working in an old pickup truck?

The best and easyest ways to tell would be to use a voltage tester/volt meter with the engine running. I would take the volt meter and hook it up to the battery and take a reading with the engine running around 1000 R.P.M. .The average reading should be around 13.50 to 14.50 volts DC (this is with a 12 volt system) the truck should have something on the dash to indacate the status of the charging system,some times its a amp gage or a bulb.If you have a bulb system,the light should be out while the engine is running. To check the bulb with the engine off,key on the bulb should be lit,this is a way to check the bulb. the bulb should also be lit while cranking. If you have a gage-the gage is either volts or amps. if the gage reads in volts the reading should be simler to the volt meter reading that was taken at the battery. If you have a amp gage-their usually have a zero in the middle of the gage, usually to the left of the zero would indacate a negitive numbernot charging,if the reading is to the right of the zero that would indacate charging(note:amp readinga are with engine running @ 1000 R.P.M.) also note the you ahould have all accories turned off.