By all means test it, but I have boring news to you: nothing special happens. Manufacturer tests them at rated power given the test procedure image shown. You can, of course, wrap some paper around this resistor and make the paper discolor or even ignite, or block the convection cooling or thermally insulate the resistor and make it fail, but
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif)
Discoloration of the resistor may or may not happen and does not indicate failure because the markings exist for part identification during assembly.
If it fails, it's big news. So far, people who talk about resistor power rating conspiracy have produced zero evidence, even when the test is really easy to arrange. Absolutely zero, not even circumstantial. If there is something to it, and anyone proves it, I'm happy to stand corrected and join your camp.
Of course, some derating is almost always a good idea. With components for professional designs, manufacturers do not much or any safety margins as that would only fudge the reality; it's a convention that this task is for the designer to do (it could be the other way, too, but this is what the convention is, and it only works when everyone follows the same convention).
If you want to criticize an almost con-like manufacturer convention, look at how tantalum capacitor voltage ratings work, and how manufacturers recommend 40% derating in separate documents because the parts are literally rated to some dozen
hours of lifetime, and after that they burst in flames violently! Another commonly used front page specification which carries no practical purpose and exists only to mislead young players are MOSFET ratings at T
j = 25degC.