Detect from browser if specific application is installed

When installing your client-side app you could modify the browser configuration to include another request header in HTTP requests and then have the server code look for that header, for example as a supported mime type using the following registry key (for Internet explorer)

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\                                                      Internet Settings\Accepted Documents

I am not sure if Opera and FF use this same key, but they likely have similar configuration options, but this should at least get you on the right track.

If you want to detect with javascript inside the browser, you can probably use the collection "navigator.plugins". It works with Firefox, Opera and Chrome but unfortunately not with IE.

Update:In FF, Opera and Chrome you can test it easily like this:

if (navigator.plugins["Adobe Acrobat"]) {// do some stuff if it is installed} else {// do some other stuff if its not installed}

Update #2:If it is an ActiveX object in IE you can test if it exists by using something like this:

function getActiveXObject(name){        try{            return new ActiveXObject(name);        }        catch(err){            return undefined;        }};

Another approach for IE is something similar to what JohnFx suggested (I found it here and have not tested it):

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\InternetSettings\User Agent\Post Platform

Well the method that was mentioned by "some" is very common but there's also other methods for example if the application is listening locally on a port it can be detected from the browser which is very common for security solutions because have some sort of local proxy to filter the traffic. I actually made a video a while ago about using this technique to detect avast anti-virus you can watch it here:

https://www.youtube.com/watch?v=39_Nd8oiEAk