On second thought, screw the line that gets traced on the wall, look at the pattern.
Example: Reflect light of a CD you're holding in your hand onto a wall. Flex the CD a little. See how it goes from a round circle of light (flat) to something funky shaped (not flat). That's using non-coherent light at an acute angle. Now imagine using coherent light and an oblique angle. Any deviation from flat would be pretty obvious.
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