Leak down test time.
I know it's at the back and it's a pain in the rear, but turn the crank by hand until you feel #4 on its compression stroke. You can get a neat little tool that whistles as in indicator - it's pretty much a necessity as it's impossible to get your finger down the spark plug well.
The leak down tester will flow regulated air into the cylinder. At this point your listening for air coming out the exhaust (exhaust valve), intake (one of the intake valves) or through the oil fill (piston rings).
2003 GMC Sierra 3500 -- dually, 4x4, hasn't blown up yet and that's what i look for in a truck
Ford Doctor -- They come in broken and leave fixed. Ford of Canada Master Tech