How to find out if your alternator konked out?

[netken]

Having this problem where if my car don't start for 2 days, i will need to jumpstart it... happen for 3 weekends in a row...

Changed the battery to AC Delco MF but still same problem... switch to another AC Delco batt but problem still persists....

However, when car starts, charging show 13.0 Volt .... sometimes when idle at traffic light.. drops to like 12.2 Volt .. and even once to 11.5 Volt ....

And when engine is off, its about 12.4V only ... is this a sign that my alternator is a goner? pls help ... problem still unsolved...

Thought it was battery drain, checked alarm, audio, etc .... can't trace ...

Only things that i have changed before this problem :

1. Single fuel pump to Double fuel pump.
2. Original 80A alternator changed to a used 120A alternator.