provenance? how too

[LowlyAssassin]

What are acceptable methods of improving the origins of a piece of art work
II don't known the name of the word

OK so I have had somebody offer me a piece of art work and its signed but its a print so not original art work

Would a letter signed by the artist be an acceptable piece of proof that it was original?
Or would I have to have it signed by the person that was giving it to me as well

The art print was a print of a drawing by a concept artist and was given to members of the game devs
Not the dev that received the print has offered it to me
So I was thinking would a letter headed piece of paper explaining the origin of the piece and signed by the dev who is giving it too me be sufficient to prove authenticity? Or would it need to be signed by the artist too?
Or do I need something else?
Answer would be good as soon as possible thanks