Personally the string method didn't work well for me. Seemed to have too much room for error.

My polaris shop manual suggests the chalk method.
Mark a chalk line on the inside of each tire (at the hub height).
Measure the distance between the chalk marks. Roll the quad forward so the chalk marks are at the rear on both tires (at the hub height again). Measure between the chalk marks again.
The rearward measurement should be 1/8 to 1/4 less then the measurement in front.
Also recommend strapping the bars so they stay straight the whole time as mentioned above.

 

Might be worth making sure the tires have the correct pressure, with the fronts matching pressure and the rear matching pressure for most accuracy.