The woman said to him, "Sir, you have nothing to draw with, and the well is deep. From where then have you that living water?
Whence then hast thou that living water ... indicates that the woman had already apprehended the fact that Jesus was not talking about the water of Jacob's well.  This question of hers reveals that she understood at least a part of what Jesus was saying to her, and that she might have suspected, even at that point, the metaphorical significance of his words, as the next verse shows.