Leaky air bladders inside the lockers (I have had one), tweeked/bent axle shaft tubes, obvious signs of hard trail usage as indicated by big or deep scratching/gouging underneath, chipped or broken ring or pinion gears, etc.
A big syringe can be used to apply air pressure to the locker's air hose fitting to test it for leakage. Caution that it only takes 5 psi to engage the locker, air pressure from a standard compressor would likely blow the bladder apart. Once you hear the locker engage, leave the syringe on to keep it pressurized & wait to listen for it to unlock which would indicate a leak.
It's obvious exterior condition will tell you a lot about how it was used... hard use would be pretty easy to determine. Good luck with it and keep in mind few axles can't be restored to good condition for a reasonable cost.