With the string method, you wrap a long length of string around the rear tire once then run one length up each side and past the front wheel. You position each piece of string so that it is just barely touching the leading edge of the rear tire and use the string as your straight edge. You then measure the distance of the string to the front tire. If you have dual front brakes, it's easier to measure to the brake rotors.
A big straight edge like a flourescent light tube or a carpenter's level is easier to work with. Personally, I trust my tape measure to be close enough and it's plenty easy to use. If I had a long straight edge handy, I would use that method, though.