People that make buildings do not work to a high level of precision. A room can be out of square by an inch and you probably wouldn't notice until you tried to put in flooring that comes in accurately cut rectangular sheets. The last pieces you add get cut to fit, not cut to whatever the dimension is supposed to be.

You can use a tape measure to measure opposite corners and compare to the other pair of corners to check squareness. You use a level to check level.