Previous solutions that once worked like the CSS history hack have since been rendered unusable by all browser vendors. To anyone else coming across this question, please note as of 2014, there is no reliable or accurate way to detect if someone is browsing in an incognito/private/safe browsing mode through Javascript or CSS.
The ability to detect private browsing mode has been acknowledged as a bug and was never intended. So, if you're wanting to detect private browsing it's now impossible (unless you find a way to do it that Google hasn't found).
Google is removing the ability to detect Private Browsing Mode permanently in Chrome 76 onwards.
