i have aquired for my use a sedgeway reticle leveler. great tool. sure takes the guesswork out of mounting scopes.
i worked at a gun shop for a while, and we always "eyeballed" the scopes we mounted, and could always get them close, but i am a little particular about my long range guns, therefore i felt the need for a little better method.
i used to shoot at 100 yards at a line on a target, and would adjust the scope impact to see if the crosshairs were square by checking bullet impact left or right. that is a little expensive, but the end result is good.

herman and bill, i sure hope your neighbors arent home while you are pointing your gun at the edge of their home. i dont think i will use that bit of advice, even with a gun with the bolt removed........lol