>communicating with a spirit and have no natural psychic power of their own
I'd say that the ability to reliably communicate with spirits qualifies as a psychic power.
>and just be using hypnosis/persuasion techniques
Hypnosis that actually works and isn't just psychotherapy is a psychic power. If you're worried about persuasion just don't let them talk.
>What's a foolproof way to make sure someone is a real psychic and not a fake?
Have them try and read the mind (I assume this is what you're talking about) of someone in another room that they haven't met. Or just determine the emotion that they're feeling.