If that's the point he's trying to make then the only thing a person can know is if they AREN'T a partaker of Christ, because if holding fast to the very end is how and when you know you ARE a partaker of Christ then you can't know that until you get to the end. I'm confident that's not what he's trying to stress.
The point is not whether you were really a true believer to begin with, or not. The point is, you are not a partaker of Christ if you fall away. Only people who believe are partakers of Christ. The person who once believed but fell away is not a partaker of Christ (anymore), and the fake believer who falls away is not a partaker of Christ either.