I would think that "eyeballing" the sight would allow one to detect variation from plumb. I've seen a few revolvers with a canted from sight, but in each case the barrel was not turned up so that it was centered. Once I sent a J frame back to Smith and complained that it shot off to one side. They fixed it by turning the barrel to cant the sight. This technique resolved the issue, but I then had a revolver with a "crooked" front sight.