Get the Classroom in a Book from PeachPit Press for the application. When you can flip through it and know every section, you're ready.
That is really the only sure method.
While there are other resources such as practice tests, study guides, links, web pages... and a whole bunch of people/businesses charging you to "prepare" for the certification tests.... which all may seem like great resources, they really aren't.
I am Adobe Certified and over the years I've reviewed most of the practice tests and study aides out there, both before and after testing. If I were looking to take the tests today, with the knowledge I have currently, I'd merely invest the $50/$60 (or whatever it is) in the Classroom in a Book go through that thoroughly and I'd feel very confident.
The Classroom in a Book (CIB) is the only true study guide since the tests strictly cover how the application functions, not what you can create with the application. Most other publications focus on helping you create the latest whiz-bang doodad while going into as little technical education as possible. Only the CIB covers every possible technical aspect of every tool and feature available in the application.
In my opinion, the "practice tests" are pretty worthless. They are just questions someone thinks are similar to the actual questions - there's zero correlation between any "practice" test and the actual tests other than they will both present multiple choice questions related to a specific application. If they serve any purpose at all, it's to alleviate any apprehension regarding the general format of questions - but never regarding any actual content.
Related: Are there any tangible benefits to becoming an Adobe Certified Expert?