if you get 14 volts with the motor running the alternator is working fine. only way to test battery is a load test for which you need a tester that will put a simulated load on the battery and check the volt drop. anyplace that sells batteries will test it for you though i have heard stories of some unscrupulous parts stores saying 'yep yer batteries toast!' when its actually not, just so they can sell you another.

Here in minnesota we get extremes of heat and humidity and sub zero temps which are the hardest on batteries, life expectancy on a vehicle that gets used year round is about 4 years here, less if you dont use it all the time. I invested in a load tester a few years ago...about $40 i think it was. Have also had good sucess with the optima spiral batteries lasting longer and taking abuse.

Pete