The fantasy is the dream that is never manifested; it remains a fantasy when no action has been taken.
It is short term. You are satisfied.
But a dream, a real dream, stays forever with you.
It does not leave you alone; it pops back in your mind at the most unexpected moments.
You may see someone else who is doing something similar to your dream, or you may pass the house or see the lovers or the car that is in your dream and you are reminded and it hurts.
This is how you can test out your dream. It matters to you.
A fantasy has no substance, no roots, whereas a dream holds within it the key to your complete happiness.