"I have one Security on staff who's authorized to use that and keeps a spell slot open.  I can order him to prepare Detect Intentions in about thirty minutes, and he can try it on you briefly."  In the form of Detect Thoughts with a metamagic Extended Duration on it, which will make the spell visibly third-circle.  "You'll need to deliberately fail a Will Save, which should also let you verify that it would take a substantially more powerful caster than him to try reading you without you noticing."

"There's more powerful mindreading spells but those would be harder to prove things to you about."

"I have one Security on staff who's authorized to use that and keeps a spell slot open.  I can order him to prepare Detect Intentions in about thirty minutes, and he can try it on you briefly."  In the form of Detect Thoughts with a metamagic Extend Duration on it, which will make the spell visibly third-circle.  "You'll need to deliberately fail a Will Save, which should also let you verify that it would take a substantially more powerful caster than him to try reading you without you noticing."

"There's more powerful mindreading spells but those would be harder to prove things to you about."