In this post Spinning considers how one should go about giving critique on a story. How the does the reader approach the work? What should the reader be looking for? Who should review?
Wed look for someone who is open, honest, and while knowledge of the art is not essential, some form of literary savvy in steering the writer towards the necessary elements of story is just as great a help as providing response as to the feelings a piece may give to the reader.
I think the answer has more to do with attitude as well as knowledge of the subject. People who love stories and talking about stories make good readers. In a workshop environment, knowing the elements and bringing an attitude of trust to the story under consideration makes for a great discussion, especially when the reader has invested in the life of the story. Best to get the writer and the reader charged, excited, ready to leave and get to the typewriter.