BUT...first you have to elevate the back of the boat enough to put the rudders down into their normal sailing possition. Usually you can do this while the boat is on the trailer, but depending on how deep your rudders are, maybe not. If I lower my trailer hitch to the ground and use a slight slope, I can just barely get my rudders all the way down and not hit the ground.

If you don't have a slope available, try lifting up the back of your boat off the trailer by putting life jackets (or something else) under the hulls where they contact the rear trailer supports, or maybe you can roll your trailer tires up onto a board to get it up off the ground.

Anyway, you must put the rudders all the way down while the boat is on dry ground, so you can do the measurements described above, unless you want to do it underwater...