I'm hesitant to prescribe a particular solution, because any number of possible situations and solutions may exist for what you are describing.

One very simple approach would be to provide some additional verification details in the `content` field of the event.  My proposal doesn't make any requirements on what must be in the `content` field, so a client for verifying a web page could give a specific URI, like "myblog.com/post1#section" to sign that specific section of the page.  The in the content of the event, a copy of the content of that section of the page could be provided.  If it changes at the source, that would be immediately apparent to the client.