It would be just a matter of time until they actually do this. I imagine it works much like their other systems in that it tries to recognize problematic / fishy code and flags it for a manual review. With enough red flags or spam reports, your site will likely get a manual review. At least that's how I've puzzled it together
- that's how I would do it
There's just no way to do an automatic detection of hidden text, it's impossible. Sometimes you can't even do it manually without really searching. Simple things like style sheets for different uses (screen, print, handheld, etc) will almost always result in several elements being invisible.
It's strange ... if you overdo it with hidden text, you'll probably be tagged for stuffing keywords into your text anyway. And if you try to "get it right" you might as well just put them into your visible part of the page. Why hide it if it'll get caught anyway - or if you could have them visible?
I wonder who's going to be the first to cloak stylesheets to Google
It could also be something simple -- like trying to check the stylesheet for compatibility with the cache-display page (but I doubt it).
Is your stylesheet blocked by the robots.txt?