Once the rider dismounts, top out the suspension and measure the distance between the zip-tie and the fork seal. Again, it's very important to make sure the fork is completely topped out to ensure an accurate measurement. When the rider is off and the fork is extended, the zip-tie indicates how much travel has been used just to support the rider's weight. Front sag should be a little lighter than rear and should fall between 1.25 to 1.5 inches, the rear should be between 1 and 1.25 inches. If there aren't preload adjustments available in your fork, it's not too difficult to create the right amount with simple materials.
Most forks have spacers between the springs and the top of the fork tubes.
Increasing or decreasing the length of the spacers affects the amount of preload in the fork tube. The longer the spacer, the greater the preload and vice versa. Many roadracers make their own spacers out of PVC tubing because it's light, easy to cut and fits nicely inside most fork tubes. Equally spaced grooves are machined along the tubes as reference points for preload adjustment. A threaded collar on the rear shock can be tightened to add preload or loosened to reduce it.