GreenReader allows you to get instant feedback on how well you are reading the green.
Simply open the app, place the phone on the green, and aim it towards the hole. The app will show you the slope horizontally and vertically. Horizontal slope informs how much a put might break left or right. Vertical slope indicates how uphill or downhill a put is.
Lastly, the app shows a suggested number of fingers that you may need to aim left or right of the hole.