I suggested to two scope company reps at a gun show some years ago that they put a spirit level in their scopes. Place it so it appears at the bottom of the field of view with the bubble centered on the vertical wire when the scope was level. Both reps though it was a good idea. US Optics has one in some of their scopes.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master