How to really stress test a responsive design?

Ive made a couple of responsive websites and everything seems to be working great. However some of the finer aspects of responsive design seem hard to test so I havnt completely got my head around them.

For instance ive read a bunch of times that ems are better for media queries and font sizes than pixels. This is coming from the pioneers and experts of responsive design, so im sure that their right.

However ive made pixel based designs and they seem to work fine. If I resize my browser and look at the site with an iPad and iPhone everything scales nicely and is readable. So how do I stress test a responsive design to find these rarer cases where my sites may break down (eg because of px based sizes not em).

Im also a bit concerned about using image sprites. These use the background position style to move the correct icon on the image into place. However if the font size changes (as ems allow it to) then the wrong or maybe even multiple icons could be visible. I also dont know how to test this as if I zoom in my browser the text and container all resize together so nothing breaks.

Basically, how can I stress test my responsive designs to the point of breaking them? Thanks