First: find a wide bay and measure two diagonal lines directly. If the diagonals are the same size, then they are squares.
Second: Measure 3(30) cm and 4 (40) cm on both sides of the angle line (90-degree angle) with a box ruler, and then measure the distance between these two marks. If the distance is 5(50) cm, then the angle is 90 degrees, which is a square.
Expect to adopt with pure hands