The very first indication that your alternator is not working is that your battery will go flat. Secondly you’re warning light on your dashboard will either not illuminate or will continuously glow while the engine is running. The best test to see if your alternator is working is to use a voltmeter. This should be connected across the positive terminal from the back of the alternator output lead (B+) and negative (B-) terminal on your battery. If you can not get to the positive terminal on the back of your alternator you can use the positive terminal (B+) on your battery. (This is not a sure safe way as there could be a voltage drop between the battery and your alternator). Your alternator should charge between 13.6 volts and 14.3 volts when idling, once your vehicle is running and you put on your accessories such as lights, wipers, heater etc, the voltage may drop to around 12.4 volts this is acceptable. If you get readings outside of these guidelines then your alternator is not functioning correctly, and should be taken to an Auto Electrical specialist who will diagnose the fault for you.