How do you know when an encrypted book is solved?

[Koen G]

I feel like when it's solved, we will know. So far all proposed solutions are so atrocious and take so little of the manuscript's properties into account, that a correct solution should be noticeably different. For example, it will hopefully not contain a step that turns the thing into a one-way cipher, or a "creative phase" where the decipherer chooses which letters to fill in until he gets something that produces a semi-acceptable word-mess in google translate.