Whether you are using a checking pushrod or a standard piece that you think might fit correctly, begin by coloring the valve tip with either a Sharpie or machinist's dye. Then, install the pushrod and rocker arm while the lifter is on the base circle of the cam. If you are using a solid lifter, set the valvetrain to zero lash. If you are running a hydraulic lifter, tighten down the rocker adjuster to your normal preload. Using a wrench on the nose of the crank, turn the motor over several times, then remove the rocker and check the mark left on the valve tip. The roller tip on the rocker should have left a shiny spot where it wore away the ink you placed on the valve tip.
All rocker designs are not...
All rocker designs are not the same-even if they are the same ratio. Both of these rockers have the same ratio. The steel unit (right) has contact points that are farther outboard from the pivot point. If you are having problems with the rocker contacting the spring retainer when the valve is closed, you may need to consider your rocker design. On our engine, the aluminum rocker on the left was allowing the pushrod to contact the guide plates; the steel rocker fixed that.
If the pushrod length is correct, this mark should be centered across the top of the valve stem. If it is too high (closer to the lifter valley), try a slightly longer pushrod or lengthen the checker. If it is too low (closer to the exhaust ports), try a slightly shorter pushrod. Now simply repeat the process, adjusting the pushrod length each time, until you have the wear mark centered on the valve tip. Sometimes the low-tech methods really are the best.
If you are using a checking pushrod, you have two options. First, you can count the revolutions the pushrod has been expanded, calculate the extra length, and then add that to the pushrod's base length to find out what length of pushrods you should order. Or, you can simply wrap a piece of tape around the threads so the length won't change, package the checking pushrod, and send it off to your preferred pushrod manufacturer. The manufacturer matches your checker against its stock and ships the correct pushrods (along with your checker) back to you. Attempting to measure the pushrod with a set of calipers is problematic because the oiling holes on either end make it difficult to find the true tip of the pushrod because it has been cut away. Most manufacturers use a custom measurement tool that most of us don't have access to, so it's really better not to even try.
When you receive your new pushrods, it's not a bad idea to repeat the checking process with the new rods. This is the easiest way to ensure a mistake hasn't been made and that you haven't been sent the wrong length of pushrods. It's easy insurance.
What About Shaft-Mounted Systems?
Shaft-mounted rocker arm systems are becoming more common in racing and require a slightly different method for determining correct pushrod length. Instead of determining rocker arm height by sliding it up and down the rocker stud, a shaft-mounted system uses shims between the head and the base of the system. Many manufacturers, such as T&D and Jesel, offer a simple tool or gauge that helps you know when the rocker pivot height is correct. After you do that, you simply use a checking pushrod to determine the distance between the pushrod cup in the rocker to the pushrod seat in the lifter (while the lifter is sitting on the base circle of the cam).
Also, Dorton recommends keeping the adjuster nut in the pushrod side of the rocker screwed in so that most of it is up inside the body of the rocker. This is the strongest position, so when you determine the pushrod length, this is definitely where you want the adjuster.
 Shaft-mounted rocker systems...  Shaft-mounted rocker systems use shims to raise or lower the rocker arms in relation to the top of the valves. The bar is part of the checking tool to determine the correct height. |  T&D includes this gauge and...  T&D includes this gauge and the bar (mounted where the rockers would go) to determine correct height. Add or remove shims until you can place one side of the checking tool on the bar and have the other side sit flush on the valve. |  Once the shaft system is correctly...  Once the shaft system is correctly shimmed, all you have to do to find the correct pushrod length is determine the length between the pushrod cup in the rocker and the pushrod seat on the lifter when it is sitting on the base circle of the cam. |