Burnt oil

You can add a small bottle of dark purple dye to the oil, then check a day or two later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If the engine is burning the oil, the dye will show up inside the tail pipe. Auto parts stores will have the dye for the fluid being checked. Those that rent or borrow tools should have a black light.

The 4.7s are known for bad PCV valves and valve stem seal failure causing them to burn oil. The PCV sticks open and the vacuum sucks in oil. The valve stem seals split and oil is drawn past them into the combustion chamber.

Easiest way to see if you're burning oil is to pull the plugs and see what they look like.