how did susan and peter discover that edmund had been in the forest of narnia before

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.

He forgot about saying him and Lucy were pretending when Lucy and him went into narnia.Well he said they head for the lamppost since the lamppost wasn't really in sight peter and Susan knew that he had been there before.