The only way to check it, is the pull the case halves apart. From there you'll have to pull the gearsets/shafts out, then pull the diff out. (If you have a helms it walks you through it). You can punch out the locating pins with a 5/32 punch (make sure you have a long punch so you don't tear up the plastic speedo ring), inspect those and then you can remove the actual pinion shafts.
For the roll pins you'll want to look for pins that are bent, or starting to bend or sheer. For the pinion shafts you want to looking for galling or severe wear.
I doubt this is something a bonyard is gonna let you do before you buy it. But you can always check after you buy. If you need diff pins you can replace.
SHO NUT has good prices on tranny parts on their website, but Ford just recently made the diff pinion kits obsolete, I don't know if SHO NUT is still selling them. I grabbed the last 3 sets I could get my hands on, and my local dealer said a dealership in OR had 1 or 2 kits left. I could get their info if you need to order them. Dealer list it's about a $42 kit.
good luck,
mark
