Plumb bob at range works, but I like to do everything in my shop. For that reason, I shine a bright flashlight from the objective end of my scope through the tube. It will reflect the shadow of the reticle on my side wall, where I have a plum string. Turn power ring down and it is perfect. I line mine up a hair off to one side, so I can see the hair sized break in the string tot he shadow all the way down the vertical retical line.
As for the level, as stated above, many of the cheaper ones are fine for what we do, I like mine on the left side of the tube, just in front of my power ring. I shoot a bipod 99% of the time, so once I have my gun level in the bi-pod on the bench, I don't need to reference the level anymore as my bi-pod is then locked in tight at level.