how does Edmund reveal that he has been to Narina before?

by accident....

"Lucy suggests that they all pretend to be Arctic explorers, but Peter responds that it isn't necessary to pretend in a place like Narnia. Edmund, forgetting that he had told Peter and Susan that he and Lucy had merely been pretending to be in Narnia, suggests that they head for the lamp-post. Peter and Susan immediately realize that Edmund has been lying to them. Peter is especially angry with his brother for having made them doubt Lucy. Edmund follows his siblings towards the lamp-post, grumbling to himself that he will make them all pay."