Google Webfonts Help

So, here I am -- thinking about using a Google Webfont on a Website -- when it occurs to me that perhaps some people will already have it installed on their systems! The brave Javascript-lover I am, I decided to use a JS-based solution to test if a user has that font installed on their system and -- accordingly -- install the font from Google webservers or not.But then it occurs to me how stupid I am; Google probably already covered that area! Well, I've decided to check that in their documents (documentation) and couldn't really find any word about that. So I ask you, dear colleagues, does Google try to see if the user has, say, Droid serif, installed on their PC?I suppose so, but there's around 50 kilobytes in question here so I'm not going to play the guessing game...