Crawl under it and look at the fuel pump under the driver side frame rail. Make sure it's not leaking (mine was). Try to take it for a night as a test drive and try to open the drain valve on the pump to see if it is full of water or not.
Also, if you can take it for a night, pull the upper fuel filter and look for rust or discoloration in the bowl and the color of the filters (black is bad). Make sure to re-prime it after you do this.
If you don't have one and are definitely going to buy a truck, go buy a Scangauge from Autozone or Oreilly's. Setup exhaust gas temps, oil temp, and coolant temps. Take it for a long haul. Check for running too cold at night downhill (no load). This will indicate a bad thermostat that might be not been seen during a hotter day drive. Make sure you pull a big hill and check the delta (difference) between the oil and coolant temps. Oil runs slightly hotter and should stay around 2-5 degrees difference. Make sure they both go back down relatively quick when you come off the hill. If you don't have a hill, really get on it for a little while on the freeway. Check that your boost it going up too. My boost hits somewhere around 35 psi when I'm on it without a load. If it hesitates or doesn't seem to go up smoothly, you might have a sticking turbo.
Most dealerships will allow you to take the vehicle for a night or two to get it inspected.
Last edited by Expirobo; 05-15-2013 at 03:01 PM.