Measuring boxes, casework and structures from corner-to-corner to check for square is standard practice for builders. But for times when only the interior corner is accessible, this Tape Tip gizmo by Lee Valley lets you get in there (and adds precisely 1" to the overall measurement, in case you need that for reasons other than squareness).
It's also got this arrowhead-shaped boss on the underside...
...that allows you to mate it with both an interior or exterior corner, if you need that little bit of help (and in the exterior-corner case, the measurement is directly read).
There is also a marking gauge functionality that I think would be tricky to use, but I can't blame the designer for trying to wring more utility out of the thing.
The gizmo runs $9.
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