This app displays the TargetAPI of all your installed apps. This lets you check which of your apps already support the latest features of your Android System.

If you are running Android 6 Marshmallow this app helps you to check which of your apps already support the new granular permission settings.

Guidelines from Google:
As new versions of Android are released some style and behaviors may change. To allow an app to take advantage of these changes and ensure that the app fits the style of each user's device an app developer should set the targetSdkVersion value to match the latest Android version available.