Following recent discussion on using tilt shift lenses could I suggest using a small electronic level. Cost about £25, or £45 if it works in two dimensions. I find these small levels much easier to use than the small hotshoe levels or the buikt in camera levels.
They are much easier to see and read and very accurate. You can even get ones to show a digital bubble from the side rather than the problem with traditional bubbles of having to look down which limits height when using a tripod. Look at digital levels on Amazon.