Not really a secret but its pretty easy way to do it. Set scope on lowest power, mounted on rifle but loose enough in rings to turn with some resistance. Stand in front of a mirror and look thru scope at the scope/bore in the mirror. The cross hair in the mirror should bissect the bore of the rifle. If it does the scope is perpendicular to the bore which essentially means it is level with the gun as it is assembled.
Tighten rings by alternating front to back side to side and finally torque ring halves to 12-15 in lb.
There are dozens of other methods but this one is the fastest, easiest and most repeatable method I have ever found.