I've tried several different methods on other cars, including the string, but not on the Spyder. None that I ever tried were really very accurate, but would tell you if something was badly wrong. Since we know how much narrower the front wheels are on the Spyder I suppose it could be allowed for.
Interesting note: A long time ago the military used a stick similiar to a broomstick to check the toe-in on Jeeps.