Forcing a page to only be allowed inside an iframe?

Question

I have a special page in a webserver that should only be shown when embedded inside a specific site as an iframe.

Using the Content-Security-Policy I can specify which sites are allowed as frame-ancestors, so putting it in an iframe in a disallowed site yields an error.

As desired putting it in the intended place in an iframe works and putting it in an unintended place gives an error.

What I want is to also show an error if the page is shown outside of any iframe.

I have not been able to find any CSP header or serverside way to achieve this.

How do I do this? Is there a serverside way to detect this (so I can return error content) or is this a frontend (javascript) thing only? If so, how do I detect this?