You just got to look at it. You can tell when a truck has been abused. Especially when you drive it...
Look at who owned it if you can. If it is some punk kid then you know where it has bee. Also look under the truck, see if it is caked with mudd...
Pull the air filter see if everything look good. If you really want to get to know a truck, look at the wheel on the turbo.. see if there is any play or the wheel is all worn out..
Get up and look down in the valley of death with a flashlight, see what is all down there. If you can't stick your hand down there and feel. If there is a lot of oil down there then there may be some issues.
Look at all the fluids. Make sure they are all nice and clean. Also if a motor is really clean up top... then somebody probably pressure washed the motor. If so, get under the motor and look up there, see how much oil and sludge is under there.
Check the blow by, open the oil filter cap and see how much pressure is coming out the cap when the truck is running.
If it has a bed mat, look under it to see how beat up the bed is.
If you can find out the history, when and what has been replaced.
There are a ton of things you can look for... Also find out if anyone nearby has AE, THAT really freaks out the owner of the truck when you hook it up to a computer... then you can pull all the codes, check out the injectors, test everything and watch everything on a test drive..