There are USED MCX8000s out there already? The thing just launched a few months ago. Sure you got the right number? LOL
Anyway, pretty much standard stuff for every controller:
– faders, buttons and knobs
– jogs
– displays and meters
– inputs and outputs
– visual inspection (scratches, dents)
Check for proper operation.
Also get a general impression of the unit. Well-cared for on the outside usually says something about the way it was treated. Stained, dusty or generally poor looking might indicate that all the mechanics have had to suffer as well as a result of “rough” treatment.