Woodworkers are all more-or-less obsessed with squareness. They have to be. Even small variances from dead-on 90 degrees tend to turn up as ugly gaps, lids that don’t fit, wonky cabinets - you name it. Consequently, most spend a fair amount of time checking angles for accuracy.
So, what’s the best way to check whether an angle is truly 90 degrees? For a small piece of squared-up stock or a small frame or box construction, a reliable try square, engineer square, or combination square is perfectly adequate. If you’re convinced that the square you’re using can be trusted,* just hold it up to the corner or corners in question and check for gaps.
To check the overall squareness of large pieces of material and large rectangular constructions, measuring cross-corners comparing the measurements is the more reliable choice.** If you’re not familiar with the method, it’s a simple two-part procedure:
First, make sure that the opposing edges of the shape are parallel. To do that, simply measure the length of each of its four sides. You should come up with two sets of identical measurements. If you don’t, you're not dealing with a true rectangle, and at least two of the corners are something other than 90 degrees.
If the length of the sides checks out, move on to step two: Measure diagonally from the tip of one corner to the other in the two possible directions and compare the results. If the two measurements the same – assuming all four sides are straight – all four corners have to be 90 degrees.
Nothin' to it. And here's a tip to make matters even easier: Treat yourself to a Square Check for Tape Measures. This clever device attaches to the end of a standard tape measure with a magnet and fits over the corner of whatever it is you’re measuring. For a mere $4 you’ll completely side-step the one unpleasant quirk of the whole cross-corners square-checking business: the annoying tendency of the tab-end of the tape measure to slip off the corner just when you’re about to take a reading.
* For more on the squareness of squares, read “Marking and Measuring Tool Basics”.
** You'll find some great tips on checking squareness and getting square cuts in this Woodweb Knowledge Base Article.