if you have the use of a volt meter connect it across your battery
with engine running at idle your looking for 13.8v - 14.2v
turn on your headlights ,heated rear screen , heated seats , aircon (high load for winter driving)
the voltage may drop below 13.8v if you then increase the revs to about 2000rpm and hold the revs
you should see the voltage has returned back to 13.8v - 14.2v
this is testing the output of the aternator
if it doesn't you will possibly need a new alternator
if it does possibly need a new battery