Is there a more accurate alternative to hot shoe bubble levels?
The topic has been discussed before
, but what ultimately remains is the unknown amount of sensor rotation. So even if you have an accurate level, and you can find a surface that's truely level with the camera's bottom plate (the hot-shoe rarely is), there can still be a small deviation. The electronic in-camera digital levels with an 'accuracy' of 1 degree are not accurate enough for really critical leveling.
BTW you can check for sensor rotation with a TS lens, by shifting left and right and trying to align the two images where they overlap. In e.g. Photoshop you can use two layers with the top layer blending mode set to difference. You'll notice that not only is (obviously) a left/right shift required, but very likely also a small vertical shift. When the sensor is slightly rotated, then the vertical shift is required as well to get closer to perfect alignment. Actually a rotation is needed, but that will require interpolation of one of the images, which will lose some pixel accuracy due to the old fashioned bicubic resampling algorithm.
When you know the rotation, you can adjust the camera level by slightly compensating with a very accurate level (set it to the negative of sensor rotation instead of 0.00 degrees).