how do you no the alternater has gone?

the light comes on and the battery goes flat.
If you want to diy test it you could try aiming the car headlights at a wall on a dark night and see if they get brighter when engine is revved or use a multimeter on the battery and monitor the voltage with the engine stopped and then with the engine running.Volts when engine is running should be higher

Steve
July 2011