Alternator is toast

Going to pick up a junkyard replacement as a bandaid until I am ready to do the ZJ upgrade.

Is there any good/easy/foolproof way to tell if a junkyard alternator is still good?

A good junk will have a test machine. Otherwise pull into the nearest auto parts store and have it tested.

About the best you can do out in the yard, is pick one that looks newer and/or the one that spins easiest and most freely.