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.
Step 1: Test for squareness
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.
Step 2: Adjusting the square
Adjusting the square is simple. Inside the channel the blade of the square sits in are two tiny islands of metal that sit proud of the bottom. It's these two islands, one on either side of the screw that holds the blade tight, that the blade itself sits on.
To adjust the squareness we adjust the height of one of these islands by wrapping the blade of the square in sand paper and grinding it down. Which island you target depends on which direction the blade of the square needs to move.
Only grind a little bit at a time and test for squareness frequently. A little grinding goes a long way and it's really hard to put the material back on once you've removed it.
Step 3: Re-test for squareness
Once your square is squared you should be able to strike the same line with it oriented in either direction. Another test is to strike two lines very close to each other and see if they're parallel.