detroit 14.0L

Basically the same things you would look for on any motor.
1) Check the coolant tank for signs of mixed oil or fuel
2) Check the fill tube and dipstick for signs of the same. See if dipstick has rusty corrosion on it.
3) Run it until it is up to temp and see if it is "breathing" to much.
4) Check for engine codes. If you have a computer to hook up to it check to see if the ECM matches the truck and the mileage.
5) Ask for maintenance records. Trucking company should have them on file and if they took care of the truck they should not mind showing them to you.
6) look for oil leaks.

I am sure there is more just all I can think of at the moment.