This is a good brand name square my father bought. When I used it I discovered it was not actually square. The rivet and brass system for attaching the blade to the handle is very good. No amount of tapping would change the alignment.
Step 1: Checking a square
If you want to check a square, you could hold it up to another square, but that one could be inaccurate, too.
The dark brown piece of wood in the photo is the end section of a piece of veneer plywood I have been carrying around for several decades. The white paper arrow points to the factory cut edge of the sheet. This makes a trustworthy straightedge. The white chipboard piece of shelving raises my work surface for this Instructable to the same height as the plywood straightedge.
Step 2: Lay down a piece of paper and tape it.
This is a piece of clean scrap paper. I taped it so it does not move.
Step 3: Score a line.
Hold the leg of the square firmly against the straightedge. With a fine point pen, mark a line along the other leg of the square for its entire length.
Step 4: Flip the square
In geometry it is true that two lines perpendicular to the same line are parallel to each other. Flip the square over and score a second line. Make it a tiny fraction of an inch away from the first line. If the square is truly square, the lines should not diverge from each other in the least.
Step 5: These lines are parallel
A close examination of the lines drawn reveals they are parallel. The square is not out of square.
But, earlier I said this square was not square when it was new. I made it square.