I asked this question before but didn't make it clear that I meant in user script, not in JavaScript from a webpage.So I'll be more clear now.
Is it possible to determine if Google Chrome is in incognito mode via a user-script (basically a script run as an extension in the browser, not a script being run on a webpage)?
If you are developing an Extension then you can use the tabs API to determine if a window/tab incognito.
More information can be found on code.google.com.
If you are just working with a webpage or a userscript, it is not easy, and it is designed to be that way. However, I have noticed that all attempts to open a database (window.database) fail when in incongnito, this is because when in incognito no trace of data is allowed to be left on the users machine.
I haven't tested it but I suspect all calls to localStorage fail too.