How do you work out what your body fat percentage is? I really have no idea how that works....

I'm a lanky skinny, bugger with a devilish metabolism so I can't see my body fat ratio being that high, but would have no idea how to score it. I'm 6'3 and and weigh in at about 76-77kg/167-169lbs (not that I would ever weigh in mind!). Not a whole lot of lard I could trim. I don't have a six pack or anything though, so I'm sure there is something there, but not a whole heap.