Originally Posted by Ahmed Aman

If the distance between the rear outermost point of the wheel is called R and at the front it's called F, with tyre diameter D it should be:

toe-in = sin-1((F-R)/(2*D))

You have to use an inverse sine (here sin-1, because the forum doesn't allow superscript), so you'll need a calculator.

OR:

You just measure the distance between the front and rear of the wheel and write that down. Should do the job just as well, with less effort. :P