Open URL - How can i check if the app URL is not available, then open the browser URL instead ?

Hi

I am making a mobile game. I would like to open URL of my page in twitter app.

How can i check if the twitter app is not available, then open the browser instead?

Thanks !

I think its explained in this thread…
Please go through towards the end…
http://forum.unity3d.com/threads/open-facebook-app-instead-safari-ios-c.143623/