As many people have said, I think it's just something you know. My biggest indicator that I'm making the right choice is that with all my previous attempts, I've felt regret that I survived, but I've never once, in fifteen years, felt regret or doubt about the idea of dying.I can't imagine a happy future, and I know I don't have the energy to try to improve my life even if I could. Pro-lifers always say, 'it will get better', but for a decade and a half my life has gotten worse and worse, to the point where there's nothing to salvage and nothing left to care about (aside from the general affection I have for my family and my dog, and a general remorse about leaving them, but it's not enough and never has been).
This is the first time I've researched and chosen a reliable method, and the first time I've spent weeks planning and preparing, rather than impulsively acting on my depression and self-loathing. So the only anxiety I feel is about someone finding out early, or surviving my attempt. Once you have that level of certainty, and find nothing to truly enjoy in your life or in your future, I think you know it's time. But it is and always will be different for everyone.