I don't know of any way to check one without pulling it apart.
I knew for sure when I couldn't get the short side back together. I had a spare long side axle sittin around so I slid it into the diff. That's when I saw that it wasn't sitting centered in the seal hole. I actually had to take the seal out because it was so far off.
The only thing I can think of is to talk to the wrecking yard, express your concern and have them mark it. Then take it home, tear it down and check it as best you can. Pull the axles out and see if they go back in easily.
I don't know if the wrecking yard would go for that, but it would be your best bet.