Also, see if it turns over evenly. What I mean is feel for "dead holes" where a cylinder has less compression than another. The motor will speed up when this happens. You can do this with a long wrench or the starter.
When turning it over with the starter does it make any weird noises? Knocks, ticks, etc...
If it does run make sure the oil and fluids look normal. No water in oil, or oil in water, etc.
This is all I did and I wound up with two running IDIs for $500. Then again, you could still get stuck with a lemon.
Pullin the valve covers would tell you somethin' but they probably won't let you.
If you are super paranoid, you could ask for an oil sample and send it to Blackstone...
Its helpful if you know the history of the engine too: Miles, or hours, Ether usage, outside storage, rebuilt, etc.
Im sure I forgot something, so hopefully someone else will chime in.
Basically if it runs well, looks decent, and doesn't cost a fortune, get it. You can check compression, and look at the glow plugs too, but if it runs, it runs.
If it's cool, you want it, and you can afford it, buy it, everything else can be fixed. Cool is hard to come by, parts are usually available. Some parts are harder to come by than others, but cool is a necessity for me. If you want a vehicle that cranks up, runs, and stops: disregard my instructions and listen to the other guys.