There are well known ways to self-prove a straight edge, combination square, or machinist square, to the accuracy needed in woodworking. Those methods only need the tool itself, a pencil or knife, a flat surface (for straight edges) or a flat surface with a true edge (for the squares.) They also amplify the error by twice, which helps the human eye spot it.
Is there a comparable way to check the squareness of a "T square"-like square? I'm thinking of things with a stock/fence that is below the plane of the blade/beam, such as an architect's drafting T-square, a drywall square, a shop made circular saw cross cut guide or router dado guide, etc.
You cannot flip them over like a combination square because then the fence is not registering against anything. Referencing against the opposite edge of a piece of wood just reproduces the same angle, not a flipped angle; even if it didn't, you are then trusting the two reference edges are parallel.
I am aware of the 3-4-5 method, but I don't feel like I can read a ruler or measuring tape to the mere thousandths that the straight edge/combo square truing methods afford. You can compare to another square, but often the square I want to test is significantly larger than my most trusted 12" square.
Is there a clever method I am not aware of?