I don't recall where I first heard the term "eyecrometer" or "Mark 1 EyeBall", but your own eye is one of the best measuring tools around. With practice i can see level, square, parallel, midpoint, one third point, grade and alignment close enough for benchwork and tracklaying, as well as other applications. I often cross-check with a tool, (usually a square, straightedge or level) but I never trust only the tool and I usually go with what my eye says.
It was an article by Andy Sperandeo where I first learnt that the key to good trackwork is getting down and looking along it. This shows up all sorts of kinks, dips and mis-alignments.