What is the definitive test for a bad alternator?

Quick history:

- Tried to start the car after it sat for over a month. Battery was flat. hooked up to CTek charger overnight. Car started fine, drove for a day no issues.
- Tried starting it again after a week, battery flat again. Replaced with an AGM from Oreilly's. All seemed fine for a couple days.
- About a week later, flat battery again. Jump started and went to Oreilly's. They tested battery said it did need charging, but the test suggested alternator may be an issue. Battery had a date sticker of 10/17...so not an issue of sitting around too long.

The only reason I am questioning a bad alternator is that there is no sort of battery warning light, dimming lights or other bad alternator signs (besides battery not charging). Am I just trying to convince myself the battery (easy fix) is the problem...and not the alternator?

 

One of the things is to check the ripple using a 'scope.

 

On occasion a diode goes south, letting your battery drain. Take the negative terminal off, when your not going to use it for for a while, a day or more. If the battery stays up to starting the car, then looks like out it comes.

 

Diode failure is quite common. If it is the diode you may be able to replace it yourself.