Best and easiest way is with calipers. They'll give the most accurate, foolproof measurement. If all you have is a ruler, wrap a piece of string around the bar then measure the length of string necessary for that as accurately as possible. Divide that number (the circumference) by 3.14159 and you'll get your diameter. Another crude method is to use an open end wrench and figure out what size fits over the tube and which doesn't. It'll get you in the ballpark at least. I've used this as a last resort when I had no better method available.