Test the alternator output, see here for info-
http://www.carbasics.co.uk/How_to_test_your_alternator.htm
Garages and kwik-fit type places will often test free of charge, it only takes a couple of minutes.
Get it tested at a couple of places if they tell you it's not working properly, they have been known to lie to get you to fit a new one!!
spud, August 2009