Nikon DSLR cameras have offered a “virtual horizon” (leveling) feature for some time now. With newer models, the feature shows left/right skew along with the addition of up/down tilt in newer models.
In my experience, all of Nikon DSLR bodies incorporate a virtual horizon feature that is not accurate enough for critical work. Other means must be used, but this is tricky.
A bubble level is about as good as the built-in virtual horizon feature— multiple bodies have shown me that the bubble level in the hot shoe or on the tripod head can be off by some amount relative to the sensor. Eyes are not enough either; the optical viewfinder is not necessarily perfectly level with the sensor! Lens distortion confuses the issue, but one can look for symmetry.
The best thing I’ve found so far is the electronic gridlines superimposed on the actual sensor image in Live View mode. But these are helpful only if the subject matter contains lines that allow one to see level-ness, such as a building or a straight horizon.
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