"There is a false saying: "How can someone who can’t save himself save others?" Supposing I have the key to your chains, why should your lock and my lock be the same?" (Nietzsche)
You don't. You try the key on the lock, and if it doesn't work you keep looking for another key. Nietzsche is saying "I FOUND A KEY! Anyone care to try it in their lock?"
Eh, that might be a bit more euphemistic than I meant to be, but the point is there.
The road to hell is paved with good intentions. How do you know you have the key?