I was thinking you meant threads poking out the top of the rim as in the liner area at first.
Yeah, I'd put it on the stand, let the air out of the tire and unthread the nipple showing the most thread. The liner should hold the nipple in place once you've backed completely off the threads(there is a recess to the nipple where there aren't threads and it varies). The difference between threads showing on that spoke minus the threads showing on the next spoke on the same side of the rim (that is still tight) will show you how many threads are actually engaging.
Being realistic, if you're running S2 rims on a bike with a Springer, I don't think you plan to go taking it off any sweet jumps. Personal opinion, 1/4" of threads engaged would be enough for regular riding conditions, considering these are heavy gauge. But if you are planning to jump it, motorize it, or make it a workhorse cargo hauler, etc., I would definitely get spokes long enough to engage threads past the start of the nipple.
I've got the same opinion about these drum brakes, they impart more torque force, sure, but even new ones don't have the force to lock up and shear parts.