Combination squares are useful inexpensive tools. Unfortunately if you picked one up at your local big box store chances are good it's not actually square. The method for testing a combination square for squareness is as simple as the fix if it's not.
To test any square for squareness all you need is a surface with a known straight edge, a sharp pencil, and your square.
Butt the square up against your straight edge and strike a line. Flip the square over and strike another line starting in the same place as the first. If your two lines diverge then your square is not square.