Will App Store programs download when I'm on a different user than the one that started?
The App Store saves its files to a temporary folder until it is completely downloaded.  This temporary download location is different for each user of the Mac.  So, it's possible (likely) that you could have two partially downloaded copies of the same application.  For Yosemite, and its multi-gb package size, this is not trivial.

You can see if the file is currently downloading in a number of ways.  2 &amp; 3 require some familiarity with Terminal.app.


Check the incoming data bandwidth with Activity Monitor
Search for any open files held by the AppStore agent via sudo lsof | grep storeage
Check to see if the file size of anything in /var/folders/XX/XXXXXXXX/C/com.apple.appstore/* is changing.  You can find the exact path (up to the C) via getconf DARWIN_USER_CACHE_DIR for the currently logged-in user.
If the app is downloading, mouse-over the Launchpad icon in the dock should show a %done status.  (I've seen this behave erratically; so, although it's the simplest, I'm listing it last)