To find it definitively:
1. Consult your owner's manual: This is the best place to start. The manual should have a diagram of the engine compartment showing the location of various sensors.
2. Use a repair manual (Chilton's, Haynes, etc.): These manuals often have detailed diagrams and instructions.
3. Search online for a diagram specific to your engine: Searching for "[your engine size] Ford F150 oil pressure sending unit location" on Google Images may yield helpful results.
4. Look closely near the oil filter: This is the most common area for the sensor's placement. It will be a small sensor with a single wire connected to it.
Caution: The engine will be hot after running, so allow it to cool completely before attempting to locate the sending unit. Be careful not to burn yourself.