Now, while you can always use the 3-4-5 Rule to determine square on any scale when you're laying out a project, there is a more precise way to determine whether your nearly-completed assembly is square.
Measure the diagonals with a tape measure and check to see if the two distances match. If they are equal, your assembly is square.
Case in point: take a look at the drawing of a raised-panel exterior door on this page. If we measure from one corner to the opposite corner diagonally (as shown by the red line), and then compare that distance to the opposite diagonal measurement (as depicted by the blue line), the two distances should match exactly. If they are equal, the assembly is square.
Now, what do you do if the two diagonal measurements don't match? Adjust the assembly. In the image above, if the red line's length is longer than the blue line's length, push inward on the two red corners. If the blue line's length is longer, push inward on the two corners of the assembly at the ends the blue line. After adjusting, cross-measure both diagonals to check for square again. Keep adjusting and cross-measuring both diagonals until the distances match, and your assembly will be square.
TIP: It goes without saying (that's why I'm going to say it) that the item you're measuring should be designed to have four right-angle (90-degree) corners. In other words, if the two long sides or the two short sides of the assembly aren't equal in length, this rule cannot be applied properly. However, any parallelogram can be squared by making the aforementioned measurements and adjustments.