The alternator can be checked using a voltmeter. Connect the red lead to the positive termainal of the battery and the black lead to the negative termanal of the battery. A good alternator will have 13-14.5 volts.
Your alternator is about to go! Get it replaced now or you could get stranded on a highway in the middle of nowhere like I did! You might have a week or 2 from the first time you see the warning message! Don't ignore it like I did....it cost me dearly!