How do you know if it's the one? Solely you fill it. You don't have the urge to try anymore on. You can picture yourself walking down the aisle or staring at your fiance in it. You see yourself in the moment.
When no one else's opinion matters you know it's the one. You feel beautiful and you can't stop thinking about it. You may not even want to get out of the gown.
So many gowns will look beautiful on you sometimes that can be confusing. However you will know because the feeling you get while in it.