Any way to tell if a used differential is good?

Seems like the diff on my NA is starting to go bad so I need to start looking for a replacement. The shitty part is I've barely even put 5k miles on this 1.6 open and it already seems to be going bad. Not sure why. I use the proper fluids, change it frequently, and don't beat on the car (apart from autox).

I was wondering if it's possible to tell if a differential is good before putting it in the car or if I just have to trust the seller. Sorry if this is a stupid question. Pretty sure the answer is gonna be no but I figured I'd just ask so I don't get shafted with this replacement