I've written about framing squares but they are generally too small to ensure that something like an entire wall or foundation line is square. So there are a couple of other methods that can be used which only require a tape measure.
The first of these is very simple. For any rectangle, the diagonal lines from one corner to the opposite corner are equal. Assuming you have a rectangular or square shape, you can just measure both diagonals and if they are equal, then the corners will be square. If one is longer than the other, then the shape is more like a diamond than a rectangle and you need to push in one of the corners on the longest diagonal until both are equal. One thing to be very careful about here especially on framing is that the sides are the same length. If the sides don't match (top is not the same length as bottom or left side is not the same as right) then the diagonal rule does not apply.
The second way is to use simple geometry. You can do this without using a calculator if you use a 3-4-5 right triangle. If you have a right triangle with sides of 3 units and 4 units, the diagonal side will be 5 units long. So start at the corner you want to check, make a mark along one side which is 3 units out (use feet or even yards or meters as the units) on the other side make a mark 4 units out from the corner and then measure the diagonal distance between the two marks. If it is 5 units, then the corner is square. If it is less, then the angle is sharper than 90 degrees. If it is more than the corner is more than 90 degrees.
Once you have the corner square, it is a good idea to lock it into place using a diagonal brace. Triangles are much more rigid than squares. If you bump the corner of a triangle it won't shift into another shape, but if you bump a square it easily turns into a diamond shape.