A kite is a quadrilateral that has 2 pairs of equal-length sides and these sides are adjacent to each other.
Hence, if they were careful about the lengths of the sides of the fabric then we are sure that the shape is a kite
Also a kite has the property that its 2 diagonals intersect each other at right angles.
Therefore, without measuring, we can be sure that the angle is a right angle and the pieces of wood are perpendicular