I've sort of tested them over the years. "Sort of" because I've never particularly cared to test scraping the screen of my iPod touch or other devices. I'm diligent, but I have my limits. Instead, I relied on videos the companies would post of hammers and electric etchers manhandling screens, and seeing those screen protected impressively. My testing was limited to how easy or difficult the products were to apply. (I would always acknowledge in my reviews what was and wasn't tested, that I was relying in part on the kindness of strangers.)
The other week, I had a brainstorm. I took a blank CD, turned it over and covered part of it with the invisibleSHIELD screen protector from a company called Zagg. (I suspect my results would be the same with others I've tested over the years, like ones from high-end companies Wrapsol and Lifeproof, since they all have equally impressive videos.)
Then I got down to business. I took out a screwdriver and began scratching across the CD. Then scratching harder. Then I really put my whole body and elbow grease into it -- not just scratching, but literally scraping the back of the CD as hard as I possibly could. Part protected, part not.
I took off the Zagg invisibleSHIELD screen protector. You can see the results below. Look on the far left.
I'd always been wary of putting my iPod in my pocket, concerned that keys or my pocket watch might scratch the screen. After doing this test personally, after digging a screwdriver as hard as I could across the back of a CD and not have a single blemish under the area covered by the screen protector...trust me, I now have zero concern about my pocket watch or scary keys simply bumping or even scratching into the protected screen. No concern. None. Nada.
I can't speak for all screen protectors, but I suspect the best of them (especially those willing to show videos in action) work just as well. These are Good Things. And I can speak well of the Zagg.