A free app for Android, by Maxcom.
This application is designed to measure angles in various ways.
The most basic and easiest method is to use your device's built-in camera. To do so, simply make sure that your device is held horizontally, and then take a photo. The app will then calculate and display the angle.
Another method is to use a magnetometer. This is a sensor that can detect the presence of a magnetic field. If the sensor is placed near a wall, it will output a voltage that varies according to the strength of the magnetic field.
A third method is to use a touch sensor. A touch sensor can detect the presence of a physical object.
A fourth method is to use a gyroscope. A gyroscope can detect the rotation of a physical object.
