Do you mean plugins such as Flash and Java? These can be enumerated with Javascript code in Gecko/WebKit browsers[1]. AFAIK there's also something similar for IE.<p>When it comes to extensions such as HTTPS Everywhere and AdBlock etc I don't think it's supposed to be possible. Some extensions may unintentionally leak information that can be used to detect them though[2]. What are you trying to do?<p>[1] <a href="https://developer.mozilla.org/en-US/docs/Web/API/NavigatorPlugins.plugins" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/API/NavigatorPl...</a>
[2] <a href="http://ha.ckers.org/blog/20060823/detecting-firefox-extentions/" rel="nofollow">http://ha.ckers.org/blog/20060823/detecting-firefox-extentio...</a>