How do you identify Contribute "browsers" from others?

I'd like to show some extra information to people who are
browsing a contribute capable site. E.g., having a hidden div with
special edit instructions at the top of the page that only shows if
the browser type is Contribute (and probably from a specific domain
/ ip range).
However, all my HTTP header splunking shows that Contribute
doesn't seem to supply any special identifying info. In fact the
user-agent header is the same as IE.
So, does anyone know a way to identify Contribute at the
server side?
Also, it would be nice if there was a way to have a locally
defined Contribute schema that would launch contribute from the web
browser (I know about the tool bar...). That way, authorized users
(in the web app sense) could see a link like:
<a href="contribute://site/currentURL" >Edit this page
in Contribute</a>
You could then be browsing a site with IE/Firefox/Opera and
click to edit, even if you have turned of the Contribute tool
bar.