This sounds like it would be largely browser dependant, but I'd imagine any mobile browser worth its salt would try to cut down on data usage by not loading images (and possibly not loading entire stylesheets) that are marked as not for it. Furthermore many mobile browsers prefer to not be recognized as mobile browsers. I know I hate it when I pop open a site on my iPad and a mobile-stylesheet forces me to view a skinny sliver of single column site on my 9.7" screen.
So media queries are unreliable, but still worthwhile (they really don't hurt anything, so long as they're used responsibly), and that doesn't help what is a fairly obtuse (but still good) question; time to do some testing!
Most modern desktop browsers come packaged with developer tools. My current favorite is FireFox's dark and pretty web inspector (the 3D view is especially to die for). But what about on Mobile? The largest parts of your mobile audience will not be on browsers that come with developement tools.
Well, you have a couple options:
- Firebug Lite has some mixed results on mobile browsers, but is
an excelent choice in most cases where other inspectors are
unavailable. It does seem to work in iOS and other mobile
browsers with HTML5 support, though.
- This question suggests using something called "weinre". I've never used it, but it looks legit enough.
- If you're okay with targetting just a few certain browsers, many DO include developer tools. Such as Google Chrome for Android!
Whatever you choose, you'll be looking for an asset viewer of some sort; perhaps a timeline view. Any sort of tool that will allow you to see what the page loaded, in what order it loaded it, and how long it took to load.
Good luck!