title: Detect from browser if specific application is installed
body: <p>We have an advanced webpage (ASP.NET, C#), and a application which needs to be installed on the client computer in order to utilize the webpage to its fullest. The application is a tray app, and has primarily two tasks. Detect when certain events happen on the webserver (for instance invited to a meeting, or notify of an upcoming meeting). The other task the trayapp has is to use a custom protocol (trayapp://) to perform some ajax calls back to the server.</p>
  
  <p>One problem we have is how to determine if the application is installed on the local machine or not. Now the user has to tick a checkbox to inform the website that the application is installed, and that it's safe to call the trayapp:// url calls.</p>
  
  <p>Is there any way, for instance through a JavaScript or similar to detect if our application is installed on the local machine?</p>
  
  <p>The check needs to work for IE, FF and Opera browsers.</p>