What type of square are you using?
For framing work and cutting wood less than 12", I like to use speed squares. I have several 6" ones made out of aluminum and one 12" one made out of orange plastic. These are very stable and do not change. I also have solid, flat, steal and aluminum 18" x 24" squares that can be bent but you really have to be very rough with them to mess them up. I occasionally check them with the method below. Be sure to use a board that was machine cut and is perfectly parallel or it will look like a perfect square is not.
The squares that are made of 2 or more pieces have to be checked on a regular basis. Toss them on the table one time and they can be out of square.
By the way, if you do not have the first two types of squares, check out your local yard sales. That is where I get most of my tools and for very little money.
Jim