You won't. 

Much better to start with a clear picture of what you want and then edit it along the way if you feel the need for it. 

Just like plans are subject to change when you come across new information, what you want is also subject to change as you come across new information. 

We're always seeking to improve, to have more and to be more. So, "I'll know it when I've found it" isn't a great strategy to go by.

Make up your mind in advance.