Question

How we get to know that magnetic field inside the bar magnet is uniform?

Answer 1

By holding a magnetic ball by a string on the same pole as the magnet

If the ball rotates in a constant velocity then the magnetic field emitted by the magnetic bar is uniform

Answer 2

The magnetic field around a magnetic dipole varies similarly to an electric dipole. It is stronger at the poles than beside the magnet by about a factor of 2.