Sure way of checking the alternator - engine warm & idling, switch on main beam, hazards, heater, rear demister.
If the lights dim as the hazards flash or the engine struggles then the alternator is on its way out, there should be little noticeable difference as you increase the rpm of the engine.
Modern alternators are supposed to be able to cope with all the rigours of modern motoring which would seem to be sitting idle in a huge traffic jam, middle of winter at night.