I use a small bubble tube like would come out of any level. Level up the rail, then put the level on the top ajusting knob area where ever it will sit right. As I tighten I double, triple, quadruple, quintuple check the gun compared to the scope...you get the idea. I've swapped scopes so much in the last 3 months on my 4 rifles that I've gotten pretty good at it.
Then I verify that it's properly done at the range.