I use one of these http://www.riflescopelevel.com/verti...nstrument.html
I imported from Brownells in the States a few years ago. As long as your scope objective is round and the top of your barrel/action just below it is also round, it will centre itself. You clamp the rifle. Then with the instrument sat on top of your action and the top bit resting on the scope bell, get the rifle so the bubble shows level, then you look through the scope (the slot allows enough light in) at a plumb line and adjust your scope so the reticle is vertical.
Sometimes when I'm lazy though I just do it by eye and seem to get pretty close. I don't look for "level" as it's too easy to confuse yourself, I just look for the bottom vertical crosshair to seem to bisect the action beneath the scope.