<p>What's the best way to detect that Adobe Acrobat Reader is installed from a web browser?  I'm assuming it would have to be done on the client-side (I don't think Adobe adds any user-agent strings).  Preferably in JavaScript and in Internet Explorer, although it would be nice if it could also be done in FireFox, Safari, etc...</p>

This article
from the Adobe website should get you what you need.
EDIT
In testing this it isn't 100% though, as I have acrobat, but it doesn't detect it in IE 6.