just wondering if the warning light on the dash stays on when you run the engine. a sign of brushes wearing out is that the light stays on and when you rev your engine, it dims and maybe even goes out.
the easiest way to check for sure if its your alternator is to charge your battery overnight with a charger than next morning do a test.....
first measure your battery voltage, this should be somewhere in the region of 12 volts, sometimes a bit less so dont worry. a really good battery will read about 12.2 volts after a full recharge and left to settle for a while.
next switch on your ignition and start your engine. the voltage should have risen to somewhere in the region of 13.8 to 14.2 volts
if this is the case you need to then turn on all of your lights and sundries, demisters etc....this puts the alternator on a full load test. ... if the rugulator is ok, the voltage should drop to about 13.4volts or somewhere there abouts. it should hold this reading for a priod of time, say 10 mins. if the voltage remains around that mark then you can say to a major degree that the alternator is ok......
sometimes a bad connection can cause a load so check that all connections to the battery are clean and clear of any amalgum(the gases can escape from the battery, sulphuric acid causes corrosion). it is wise to use a grease/ petrolium jelly such as vaseline over your battery terminals to prevent this from happening
i hope this helps you to some degree.....