It will depend on how your truck is equiped. A 2005 4X4 F150 can tow anywhere between 6000 lbs. and 9500 lbs. depending on engine size, axle ratio and wheelbase. Check this guide.
https://www.fleet.ford.com/showroom/.../2005_F150.pdf
Check the UVW (from sticker inside cabinet) of the trailer that you are looking at and add approx. 1000 lbs. (gear, food, clothes, etc.) That will give you an estimate of what you will be towing.