When testing alternator, how do I know when battery is completely charged?

get battery charged before testing alternator if it is low,below 12.6 vdc.check battery voltage with car off,should be 12.6,then check battery voltage with car running,should be 13.5-14.5.that means the alternator is working.the alternator don't charge the battery,only maintains the battery charge.battery is for starting car,then alternator takes over.