FOR ANDROID STUDIO 4.2 OR GREATER
1. File
2. Settings
3. Experimental
4. Unchecked the 'Do not build gradle task list during gradle sync' 
        and click 'OK'
5. Go to file
6. Sync project with Gradle files
7. You will see the option on Signing reports under: 
        Gradle -> Tasks ->Android -> Signing Report.