how do i know for sure that the alternator is bad

oh damn that sucks i did not know that thank you for the info

You can test the alternator output voltage using a volt meter connected to the battery terminals .. Battery needs to be fully charged before testing .. Volt meter set to DC - 20 volt scale, engine running voltage should be around 13 to 14 volts.. IF not familiar with how to do this test, seek qualified hands-on help .. You may have a car savvy buddy that'll help... I prefer on vehicle testing as opposed to removing the alternator for some parts store salesperson to do the testing on some sort of in house machine .. IF it test ok at the store, that simply means it was working at the time it was on so called testing equipment! .. *Intermittent failure* may be difficult to diagnose.