with engine on idle switch on rear heated window, lights and operate leccy windows - this will put a load on the alternator and you should hear the engine note change as idle decreases slightly. I would say if you hear no change then the alternator has failed.

The proper way is to measure voltage at the battery with engine off, you will get about 12v, with engine running you will see 13.5v ish, but I realise not everyone has a multimeter to hand for this.