Is there a spell, ability or magic item that will let a PC identify who wrote a letter?

In my campaign my players are working for a secretive organisation, and gets their orders through a magical scroll that lets them send messages back and forth. There are two copies of the scroll, and a copy will display whatever is written in the other.

They now suspect that someone is impersonating their contact, and want to try to locate either the other copy of the scroll, or somehow identify the person writing in the scroll.

Is there a spell, ability, or magic item that will let a PC do this?

So far I’ve identified Legend Lore, which could probably get them some information on the organisation they’re working for, and Scrying, which (depending on how you rule) could let them target the sender. There is also the Retriever, even if it feels a little extreme.

(I could just homebrew something, but it is interesting for me to know if such a thing already exists)