Most people suppose divine revelation to come as if by something akin to an hallucination - in other words seeing visions or hearing voices with a divine origin. Or revelations may happen in a lucid dream, or even by conversation with an angel.
But another kind of revelation comes via the imagination - a direct communication from the divine that 'appears' in the imagination with no sensory intermediary.
But how do you know this has happened? What follows is from personal experience - for what it's worth...
In brief, while reflecting or pondering, you become aware that thinking, the stream of consciousness, has been affected by the revelation. An idea is now in the mind that was not there before. Or, perhaps you may be writing and you become aware that your writing has been shaped.
Or, in prayer, you achieve a stillness, a receptive quietness, and then you become aware that you have an answer. This answer is now perceptible in thought.
However, you are not aware of this thought actually arriving in the mind. It has grown in the mind, but you were not aware of it growing. It is known retrospectively.
So, imagination is a kind of transformation, a change in consciousness, or an impression made on the mind; but if you are not introspecting, if you are not looking within with an uncluttered mind - then you will not be aware that it has happened. And indeed, it may fade from the mind without having been registered.
I regard such revelations as being of core importance to the Christian life, essential for indomitable faith - but in the busyness and distraction of modern life, or by focusing on the spectacular 'hallucinatory' type of revelation, it is easy to suppose that someone might never notice revelations via imagination.