A system of levels and/or wedges will do well enough in many cases but they work on the assumption that everything is mechanically perfect.
Turns out the only tool you need is a mirror...
The requirements to eliminate cant error are:
1. Align the scope's reticle with the rifle's barrel.
2. Hold the reticle level when shooting.
Item 1 deals with scope cant…i.e. mounting the scope incorrectly.
Item 2 deals with gun cant…i.e. holding the gun incorrectly.
To correctly align the scope to the barrel, set up a mirror at a distance of, say, 5 yards and set your AO to 10 yards. Look through the scope at your reflection in the mirror. Twist the scope in the mounts until the vertical bar of the reticle simultaneously bisects both the muzzle and objective bell. Then lock it down. At this point you have eliminated scope cant (item 1). If it's hard to see your muzzle, add a little dot of White Out or take a dot from a hole punch and tape it to the muzzle with clear tape.
Now to eliminate rifle cant, install and use a level. View a known good plumb line (e.g. hanging string) through the scope and orient the rifle so it precisely aligns to it. Now affix your level so it indicates level. From now on when you're shooting, hold the rifle so it shows level and you can be sure you've eliminated both potential sources of cant error. (Note that a level which attaches to the rail will almost never be correct unless it has a separate fine adjustment, whereas a level that attaches to the scope tube can simply be rotated to the proper position and locked down.)