Use your devices magnetometer, and read out the magnetic field (micro teslas).
You can observe and examine the magnetic fields around you.
You will find that certain materials change the magnetic field. Such as Iron, magnets, nails, speakers, some metals, electronics, the earth, and more.
This may be used to find things, that are ferromagnetic.
Change the feedback level to get a vibration when some strong magnetic field is nearby.
Observe the graph to visualize the fields around you.
Can be used in construction: by attaching a magnet to something behind a wall and use the app to locate the target.
The magnetometer, is usually used in navigation to detect the magnetic fields of the earth.
The phone must not be in mute for the sound to work.