Testing Aftermarket Alternators at Parts Store

I took my Powermaster 75a (60a/99a) alternator into NAPA this afternoon to test it. Since that unit was not in their computer, we picked the highest rated unit that we found on the list (65a). The test was attempted three times and each time it told him to check the grounds. So I walked out with no answer.

Is there something the parts stores can do to test an aftermarket alternator if it is not listed on their computer?

Thanks

It still is a alt that was used on a car at some point, my denso mini is basically a late 80' Toyota alt, if it's a mopar one have them test it as a late 70's or so diplomat or something.

For reference, we tried 74 challenger 360 for the below car.

EDIT:
I looked up a 79 Dodge Magnum and it showed alternators for 100 or 120v. I will try that next.

I just went through the same thing and they could not run a test on it. If they can't put in the parameters by knowing what car it came off of then the computer will not read it

The older machines would read the output of any alternator but not the new ones - that's progress right?

Actually, it is progress. Alternators have different parameters depending on how designed and built. Knowing what you are testing will tell you if it meets the parameters and not rely on some parts store droid guessing it's ok. If you want more than that going to the parts store is the wrong choice, take to an alternator rebuild shop.