The 2 most immediate tipoffs will be the transmission in shifts from 2nd to 3rd - mine never grinds, but if rushed, I can detect a SLIGHT tendency to "catch" 3rd gear - as thought it ALMOST wanted to grind - and of course, the common tendency for these trannies to be stubborn to get into reverse sometimes...
As far as the differential - about all detectable there, will be the apparent operation of the LS portion to function as it should - I expect it to display SOME tendency to spin the inside wheel in tight turns on loose surfaces - but not to grab or chatter on asphalt. Other than that, not much to tell externally unless regular oil analysis is done, and unless something clearly visible in occasional small samples drawn off look suspicious, probably won't have the lube analyzed.
My transmission stock lube was exceptionally clean at 6000 miles - so didn't even pull the cover to check any magnets - seemed a waste - but the differential lube was VERY black and heavy - probably fine LS clutch dust suspended in the lube. I cleaned the innards of the differential as thoroughly as possible, and while the cover was off, drilled and tapped it for a plug so future changes could be made without removing the cover.