Regardless of their price, the commercial squares are not quite accurate. For example, to buy an accurate square I searched through a range of 4 prices and only found one that was really good, after a dozen of squares checked. The funny thing was that the accurate square was the cheapest, confirming that it does not matter the price as it matters the good luck and the patience to check a lot of squares in the store.
In order to be able to check a square, we need a board with a straight edge and a pencil (and the square that we are checking out, obviously).
We place the short edge of the square on the edge of the board and mark a line with the pencil. Then we flip the square in the other direction and draw another line, making sure that the top end of the line overlaps with the top of the first one.
If the two lines overlap across the entire length of the square, it means that the square is accurate, regarding it’s perpendicularity. If the two lines do not overlap, it means that the square has a deviation and will not indicate as it should, resulting in a very large source of frustration, a lot of ruined material and poorly executed projects.