To find it, you'll likely need to:
* Clean the engine bay: Dirt and grime can obscure the number.
* Look closely: The number is usually cast into the metal of the engine block, and may be stamped or etched.
* Use a flashlight: This will help illuminate the area.
* Consult a repair manual: A Ford F-150 repair manual specific to your engine will show a diagram of the serial number location.
It's also possible the number is partially obscured by components like the alternator or power steering pump. You may need to move some things carefully to gain better access. If you can't find it, a Ford dealership or mechanic can help.