hi chaps

just wondering is it possible to check these boxes for condition whilst out on the ground .i have 3 spare[i never broke a box in my life lol]that i have picked up over the 3 years will possibly sell on but dont want to sell these to find out they have a rumbling beaing or bad syncro etc .

can gearbox centres check these over ?or is their anything i can do myself

all advice appreciated