One of the easier ways to create a 90-degree angle is with a triangle whose sides are in the ratio of 3:4:5. It will create a perfect 90-degree angle. You could make it out of anything, 2x4's cardboard, whatever. The longer you make them, the easier it would be to compare to your existing lines. 6'x8'x10' or anything with the 3:4:5 ratio will work.
If you are going to do this a lot, you can buy lasers that create beams 90-degrees.