How do you plan an idea's financial viability before starting to work on it?

Hey guys,

In your experiences, what's been the best way to plan out / assume / play with the finances before actually starting work on an idea?

Of course I realize that this is not set in stone, neither am I suggesting it be, I am just wondering what methods / templates you are using to asses your early financial assumptions about a product and what the big picture looks like before starting the hard work.