The prompter-focused version of alignment is the most dangerous version. If a person asks it to create a bioweapons or hack NORAD, I'd expect nearly everyone to want an "aligned" model to refuse.
(I'm also not sure the alignment problem is even possible to fully solve.)
How do you prove the alignment problem is solved?
It's directly equivalent to asking this question of a human:
"How do I know this human I'm talking with now really is a nice person, and isn't just pretending to be nice to take advantage of me in future?"
In short you can't ever really prove it. You can only be careful and judge on past behavior, and expand trust carefully. As for humans, so for AI.