You need a volt meter.
Test battery voltage with key off
12.3 to 12.8 volts is what you need
Start engine, check battery voltage again
13.5 to 14.8volts is what you need
After idling for 2 or 3 minutes voltage should come down to 13.5-13.6volts
If this isn't happening then with engine off but key on check the green wire on the 3 wire plug on the alternator, it should have 12volts(battery voltage)
This 12volts comes from the key switch via the Voltage meter on the dash.
This 12volts turns the alternator ON, yes alternators have on/off switch.
Next to the green wire there should be a White wire, it should be a short wire run to another connection on the alternator, make sure this wire isn't frayed or loose at either end.
Next to the white wire is the Yellow wire, it will connect to the Big Red wire coming from the back of the alternator, this connection might be at the alternator or at the starter Relay on the fender.
Follow the Big Red wire, this one sends out the 13.5-14.5volts, make sure the wire is in good condition, at the opposite end from alternator this wire will be connected to a fuse or Fusible link(slightly smaller wire) and then to the "+" battery cable, usually on the Starter Relay post.
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