Checking a square's squareness is an integral part of accurate woodworking, and one most of us probably don't do as often as is wise.
It's also easy. Locate a board. Place the square against it, and make a mark. Flip the square so the inside bottom edge is at the mark previously made. Draw another mark. If the lines match all the way out, then the square is square. If not, give the blade a tap to move it into line. Tap gently, but keep tapping until the set of marks made after the tap lines up.
After that, treat the square gently and it will hold its accuracy for a long time (this treatment does not work well with small engineer's squares, which are difficult to impossible to realign).