* Check the sticker: Look for the sticker on the driver's side doorjamb. This sticker will list the GVWR (Gross Vehicle Weight Rating), the GAWR (Gross Axle Weight Rating - front and rear), and the payload capacity. The towing capacity will *likely* also be listed, but if not...
* Consult your owner's manual: Your owner's manual should have a section specifying towing capacities for various configurations of your truck. This is the most reliable source.
Without knowing the engine size and specific options of your 1998 F-150, I cannot provide a reliable towing capacity figure. Estimates found online will be unreliable and possibly unsafe. Always defer to the information provided on the vehicle's sticker and owner's manual.