Run a multimeter (if you don't own one buy a 12 dollar jobie from supercheap, looks like a kids toy but it works as good as my couple hundred dollar job for basic work) on the battery to check the charge of the battery and then with the power on. If the alternator is doing its job charging the car around 13.7 volts or more, at idle your alternator is fine. Anything less and it could be starting to die. Alternators in Euros are known for short life spans.
If you are running a large sound system it can be to much for a stock alternator.
The older a car gets the more important it gets to make sure all the earth points, the ones for the battery, the lights etc are clean of corrosion and or paint.