Try her trust, push it a little. Make her promise to keep a secret (one that won't do any harm to your reputation, of course, a lie would work) and then try to push the circumstances (while she's in the dark) so that she likely tell the secret to a great number of people. If she resists the temptation, well, you can trust her then, eh?

Mercy alive, that's not wot I mean at all, Majesty. Bless my eyes, I'm not suggesting you kill her. It's a phrase only. Means put to the test in the toughest circumstances.