You will never be 100% ready, and luck (or lack thereof) will always play a role - this is real life after all.
From a pure 'case interviewing' perspective, I think you are ready when you can lay out a framework, follow it, give interim conclusions and a solid final conclusion even in a state of high stress or sleep deprivation: at that point, your brain can run the case on auto pilot, and you only have to really focus on cracking the actual case without having to worry about the mechanics.
For me, I knew I was ready after listening to LOMS and knowing in advance whether what the candidate was saying was right or why it wasn't (and how to do better); a few subsequent mock interviews with MBB consultants / coaches confirmed it - and my subsequent admission to BCG proved it.
As others have already pointed out already, the first big hurdle is to actually get an interview - and you will likely have to apply before you are ready. Good luck!