It hasn't come up in this thread, so I thought this might be worth posting.
You don't actually need a "reference" square to check the accuracy of a square. All you need is one nice straight edge, and a pen, pencil, knife, or scribe.
Paul Sellars explains how to do it
here.
The first part of Sellar's blog is about how to get a nice straight edge if you don't already have one. And he's actually incorrect that it only takes two edges to prove that both are straight .. it takes three to prove it conclusively.
But regardless, once you have a known straight edge, checking the square is super-easy.