Callback- Finding the signer associated with the "session_user" identifier

I have looked all through the SDK and the REST API, and I can't find an answer to this. Could you please assist me? In your notifications you send us a package id, a session user id, and the event name. Please let me know how I use the session user id to retrieve information about the user you are referring to. I would like to retrieve the custom id I set for that user.