Squaring a gantry - without a square?

[Instance]

There must be a more accurate way to square a gantry then just using a carpenters square and a dial gauge. Anyone with good math intuition have ideas?

[ger21]

Cut some square blocks and measure the diagonals.
Or, line them up against a straight edge, and check if the edges are aligned. Flip one over and see if it's still aligned.

[dkirtley]

String from the middle of the opposite corners of the gantry to the rails

I would check it from both ends.

(An illustration for measuring a fence)

Also, the 2-4-5 method. A triangle with the sides length 3, 4, and 5 will be a right angle.

[RomanLini]

I've got one of those 2 foot long painted steel carpenters squares, the cheap ones from the hardware store. It is surprisingly good, as good as my engineers squares! I think the big die they used to punch it out was cut on an expensive machine.

And then I use Ger21's method, cut 2 squares as large as possible and flip one over.