In Ibis form, Renatus searches the letter for signs of scratches at the end of the letter that would reveal the message was written using a quill. if there's any, he tries to see if the scratches continue after the end of the sentence that would mean that she used some king of invisible ink.
(perception 2 + awareness 4 +dice 4 =10)
If he finds scratches, he tries to cast spells to reveal the hidden message (muto aquam ?)
If he finds nothings, he shows the letter to the Jerbiton.
(EDIT by Arthur: Moved the post to a new topic, since the other thread is only for the letters themselves. I'd like to keep it that way. )