Example 1. Squares and Parallels
Ensuring 90° between two structural angles is fairly easy by attaching a third short angle at 90° to them. In most cases this will work just fine. The profile of aluminum angle is actually a tad less than 90°, so if alignment is critical, use a try square.
To make sure that two angles 2" away from each other are parallel, place 2 angles in between. For 4" away, place 4 angles, etc. If they are farther apart, use measuring tape and make sure the distance between the ends is the same.