This is a commercial vehicle 2000 peterbilt i want to know how i can check the alternator to see if it needs to be replaced?

You will need a voltmeter,or the volt gauge in the dash, if it has readable numbers on it.(test is more accurate with a voltmeter)With a volt meter,hook up positive to positive and negative to negative,right at the batteries will suffice.Make sure the meter is set to D.C. in the range of approximately 20 volts.With the truck running at idle, batteries charged,and all accessories turned off you should have about 14 volts give or take .5 (1/2) a volt,will vary depending on the charge in the batteries.If you are only showing 12 volts or less the alternator is not charging or(you have a bad connection,bad battery or a short somewhere).If it is between 12 volts and 13.5 either the batteries are not fully charged,the alternator is not charging properly or you may have a bad battery or bad connection.If the batteries charge okay with a portable charger it is not always safe to assume that they are in good condition.If you suspect a bad battery have them load tested especialy if they are over 3 years old. (It is important that all connections are clean especially the battery posts and connectors)Also if an alternator has a bad diode,it can still charge but can drain your batteries down when the truck is shut off.