Eh, Xandra is going to be under watch until she dies.

She can't really prove her role, because even though a role like that would provable, the success rate that she's given is so abysmal that it has absolutely no use. I just don't see Okamoto putting a role into the game that virtually doesn't do anything, and even when it IS successful, is rather pointless.
Three things that I know I (and probably others) keep in mind when making fake roles:
1. It's plausible -- it has to have some viable use in the game
2. It can't or is very difficult to prove to others (if you're not planning on doing number 3) Eg. Plain Innocent, Shield Raiser, Gambler, some types of protector roles, etc.
If you have the powers to do so:
3. It supports powers that your actual role has (optional, but it's a way to actually prove your role, if need be. Say you have a daykill, limited use protection, dreamer, etc. and put a role with it)
Easily enough, while that role halfway passes number one, it's DEFINITELY not provable, and from what I've been told, she hasn't proven anything yet, though people have been trying to the last day or two. I'm guessing, the terrible "success rate" is the crutch for which she can carry on not being "successful" with her role. Also, she can claim that she DID get a result and the proceed to tell a person her results, with only a minor chance of being wrong (I was thinking a 50-50 chance, but the chances are better than that)
I think I forgot to put something in... but as far as I've been told, her word is the only thing to trust, but as the amount of people that can possibly be the Poisoner dwindle down, suspicion toward her is going to go up (until the Poisoner dies). Not that it would prove she's not evil, but at least the proof that she was in the same room as the Poisoner would then be null.