I use the B-Square model.
B-Square Weaver-style bubble level
I mounted them in conjunction with mounting the scope but they can be mounted post scope installation.
When I mount them I first determine level on the rifle as best I can, them mount the bubble level. One the level(s) are in place I set the scope I place and plumb the reticle to the level.
When mounting the level also make sure it's perpendicular to the boreline as well as level with the rifle. If it's not perpendicular to the boreline you will induce an apparent cant as you shoot at angles.