How to know app(from AppStore) is still installing and didn't just hang?
One good way to tell, especially for large apps like Xcode that take some time to install / upgrade, is to open the Activity Monitor (under Applications > Utilities).   You'll find a process called storedownloadd.   You can check to see it's CPU usage to give you an idea that it's happily working away.    

You can see in my screenshot, I just started an upgrade of Xcode, and my process is running around 60%.   After a couple minutes (after this screenshot), the process dropped down to 5%, but it still working away...   as long as there is CPU usage here, you know it's still in progress.