Hi_Vel, here on the Fire, developed the Reticle Tru.
To me it's about the easiest method I've seen.
http://parabola-llc.com/DF
+1
If I can't use feeler gauges on a rail, the RT is what I use.
They provide you with all their cute quotes about quality so they can charge you $65 for a piece of plastic and a rubber band!