Question

How do you know that some entity is a form of energy

Answer 1

So it means energy is something which can itself do work or allow us to do work.Heat,which is a form of energy can do thermal work.Sound can make things vibrate.... Other energies as chemical energy have hidden effects.
hope it may help you

Answer 2

Answer:

If something is capable of doing work then that entity is assumed to have energy be it in any form.

Example:

Heat is a form of energy which makes the air particles move. Similarly sound is a form of energy which makes them vibrate.