Question

How can you tell that the magnetic field gets weaker farther from the magnet?

Answer 1

Use a compass as described below.

Explanation:

Take a compass and place it a considerable distance to the East of the magnet. Note how much the compass needle deflects from North because of the magnet as you move the compass closer to the magnet.

The deflection of the needle is an indication of the relative strength of the magnet's magnetic field compared to the Earth's magnetic field.

I hope this helps,
Steve