Before you buy it take the dipstick out a drop a dot of oil on a plain piece of white paper. If it "wets out" a big circle that is mostly clear, there is a lot of fuel in the oil-a bad sign. If it "wets out" a smaller circle that is dark in color (dirty oil color), then there is little or no fuel in the oil.
Ask for maintenance receipts. Check the fuel filter and housing-is there a bunch of gunk in the bottom?
Was it driven daily for just a short distance or 20-30 minute trips? If it wasn't allowed to warm up on a regular basis that could cause problems.
The list goes on and on...
Also do a LOT of research on that model and engine- Powerstroke Nation, The Diesel Stop, etc.