It depends on what the user wants them to see.
The psychic paper is not limited to clearance permits – it can show whatever the user is thinking at the time.
It often takes the form of a permit or ID card because it’s convenient. The user thinks “show this person something that will persuade them to show me sekrit thing”, and the psychic paper shows the identification or permit of a ranking officer. But in practice, it can show many other things – witness messages sent to the Doctor, or Jack and Rose’s flirting, or as a payment card on a London bus.
In the case of somebody who doesn’t have a superior, I’m sure the paper is inventive enough to come up with alternatives. From the top of my head:
- Identification for an inferior but very high rank that establishes them as a trusted person, if not able to give the target a binding order
- A statement of trustworthiness, possibly from a predecessor, person of equivalent rank to the target, or a friend of the target
- A fact that would persuade the target that the user is somebody worth taking seriously (for example, an obscure personal fact)
It would depend on the exact circumstances in which the psychic paper was being used.
Edit: It’s also worth remembering that the psychic paper doesn’t always work. There have been people smart enough to see through it, or with a “serious lack of imagination”. If you have a tinpot dictator who truly believes they answer to nobody, and is dead set against listening to the user, you may find the psychic paper just fails.