A lot of the lines are two-toned. This helps by letting you know where the head is and where the head ends. If there isn't two tones to your line, you can look at the specs of the line on the package that it came in and get the head length. Then, measure that length on your fly line and mark it with a marker of some kind. Taking Fred's knowledge and adding the leader length (assuming that you have a 9' leader), you're already starting with about a 40' cast!