* Your owner's manual: This is the best source for this information.
* The sticker on your driver's side doorjamb: This often lists tire size and other specifications, sometimes including bolt pattern.
* The wheels themselves: The bolt pattern is usually stamped or cast into the wheel.
Without knowing the specific trim and wheel size of your 2003 F-150, 5x5.5" is the most common and likely answer, but verify it using one of the methods above.