Towing capacity depends on three factors: 1) which engine 2) which transmission 3) what rear axle gear ratio.
If looking at a new one, Ford dealer has a TRAILER TOWING GUIDE which can tell you to the lb. for vehicle you are looking at. If looking at a used one, first check glovebox for original window sticker. This would have this info, which you could match to Guide. If no sticker found, locate "AXLE CODE" on door plate, reference it to axle code table found in owners manual and again, go back to trailer tow guide.
Sorry to make this so complex, but that's the way to get it right for any individual Ford truck product.As a general guideline, Manual transmision with smallest engine is rated for the least towing weight, Auto trans and largest V6 gives maximun towing rating.