I've never known publishers ask for proof that someone was sexually assaulted.

Most fabricated memoirs are easy to spot because there's not enough balance in them, and the details don't add up. Few people are good enough writers to write a memoir convincingly if it didn't happen.

We might well need more memoirs discussing rape and assault, but publishers won't be interested in it unless there's a market for it as well. That's going to be your problem. Right now there are hundreds of memoirs of sexual assault coming in to publishers and while they are all harrowing stories, very few are going to be published because they're just not commercial enough, I'm afraid.

Write your memoir. Make it as good as you can and of course, be honest and true all the way through. But also consider what your market might be: who might buy it, why they might buy it. Consider what separates your story from everyone else's, and focus on that. It could be a matter of structure, or it might be that your story is more extreme than most, or it might be some level of notoriety which is attached to your tale.