What most sites look at is your browser's user agent string. You can see your Firefox string in the "help >> about" menu, for example.
I really have no idea about how to modify it in Chrome, but I found this link. Apparently they offer a command-line option you can use to set it during startup.
http://groups.google.com/a/chromium....d8d830559e1d9e
I also found this interesting page that breaks down the various elements of a user agent string you give it, starting with the one your browser is using, apparently.
http://www.useragentstring.com/
And of course there's always Wikipedia.
http://en.wikipedia.org/wiki/User_agent
