Remove your secondary filter, cycle the key to fill the housing up to the top, unplug the fuel pump and crank the motor. If you see bubbles then you have combustion gasses entering the fuel system. This will give you a truck that runs horrible.
Here's a link that lays it out nicely for you. The only thing that I disagree with is instead of removing the glow plugs one at a time I recommend removing all 8 glow plugs and then moving one glow plug through all 8 holes one at a time to find the culprit. This will help you better in the instance of more than one injector leaking.