As others have already answered - you can't.
However, if you have push notifications enabled in your app, you can get a very rough idea by using APN feedback service to see which apn tokens have been removed (assumed uninstall). There is more info in this SO post: "Push notification" - feedback, uninstall application
Again, this should only be used to give a very ~rough idea on your uninstalls since the user may opt out of push notifications or the user's token changes for whatever reason.
