Especially when we are new to meditation, this question constantly arises. Here is Sri Chinmoy’s answer:

“We can easily know whether we are meditating well or not just by the way we feel and see and think. Right after our meditation, if we have a good feeling for the world, then we know our meditation was good. If we see the world in a loving way in spite of its imperfections, if we can love the world even while seeing its teeming imperfections, then we know that our meditation was good. And if we have a dynamic feeling right after meditation, if we feel that we came into the world to do something, to become something, this indicates that we have done a good meditation.

“But the easiest way to know if we have had a good meditation is to feel whether Peace, Light, Love and Delight are coming to the fore from within. Each time Light comes forward, or Love comes forward, or Peace or Delight comes forward, the whole body will be surcharged with that divine quality. When we have this experience, we know that we have done a very good meditation. Each time they come to the fore, we are bound to feel that we are remembering a forgotten story. It is only through meditation that we can remember our forgotten story. This story was written by the seeker himself, by the seeker in us. The story was not written by somebody else. It is our own creation, but we have forgotten it, and it is meditation that brings it back. When we remember this story we are overjoyed that we have created such a beautiful story and that this is our life story.”
– Sri Chinmoy