Rule 1421 A writing may be authenticated by evidence that the writing refers to or states matters that are unlikely to be known to anyone other than the person …

If you want to prove that a certain person wrote a letter or document, one way to do it is by showing the document contains information that only that person would know. The idea is that if the writing mentions private or specific details that a stranger would not know, it is likely that the claimed author actually wrote it.

For example, if a letter talks about personal events from someone's life, private conversations, or other details only that person would have known, a judge can use that as proof the person wrote it. You do not always need a signature or a witness to prove who wrote something.