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Tools by which to do this programmatically are, admittedly, a little limited.  Is this something you just need to do once, or is it something that you need in the form of a dynamic, on-demand tool?  If it's just once, I'd suggest you could create a temporary "helper app", add the original contents of the main app, then filter out and remove any objects that aren't a KC - leaving you with a helper app containing just KC objects.

Thanks Mike for response. With suggested approach also i will be left with same question, how do I fetch KC from helper app and display it on interface ?

Another (not foolproof but pretty straightforward) method to list your KCs is to navigate to the 'All Objects' view in Designer, select 'Folder' and then search using 'KC'. What this won't tell you (without some additional effort) is what Application(s) a KC is associated with or even if that KC is being used in anger. 

What's the requirement behind the requirement? Why do you need to know and what are you going to do with the information once you've derived it?

AFAIK there's no direct way to fetch objects that belong to a particular app in a procedural way with normal expression logic.