Without being able to do a compression test, etc..., I would make sure that the motor is cold before you try to start it. If you show up and the motor is already warm, I would be suspicious that it may have glow plug system problems. Make sure there's no signs of diesel leakage from the pump area or from around the injectors.
ALso make sure that the starter turns over nice and fast. A diesel needs a strong fast starter- if the engine turns over slowly it may mean that the starter is on the way out and trust me when I tell you that a good 6.2 starter is not cheap.
If it hasn't been warmed up before you show up, and it fires right up, chances are there's no problems with the GP system. The exhaust might blow a little grey smoke at startup, which will disappear as the motor warms up. If the truck is still equipped with the factory GP controller, make sure that you can hear the GP relay "clicking" under the hood during the first minute after starting the truck. It is normal for the GPs to cycle for a minute or so as the engine warms up.
If you want to know if the owner has been keeping up with his maintenance, pop off the air filter cover and check the filter. It should be relatively clean. A dirty air filter might be an indication that he isn't too religious with his maintenance.
If it has AC or cruise, make sure that you are able to switch between floor/vent/heat and that the cruise is functional. If none of these work then chances are the vacuum pump is shot. It's a relatively cheap and easy fix, but he might not know that....
Other than that, driveability issues are pretty much the same as a gasser.
Hope that helps, brother!!! /forums/images/graemlins/waytogo.gif