The method I use is simple and quite accurate. I get a couple of vials( can be had from old torpedo/carpenter levels)
and put the rifle in a gun cradle/rest and level the gun with the world. The level can be layed on the top of the receiver, layed on action rails where they are even, across the scope mounts/bases etc. and the other can be layed on the vertical scope turret, either with the cap on or off, whatever is the flatest. Simply get the 2 levels to read the same and the reticle will be absolutely level with rifle/shotgun it's on.

Here is a picture that explains it a little clearer. I have one level on the action where it is parallel and the other sitting atop the elevation turret.





***note*** When using levels in this way, it's not nessecary to have the firearm level with the world,it's just that the 2 levels need read the same.