What does Sal's mother mean when she says she wants to visit her cousin in Idaho because"she'll tell me what I'm really like"? (in Walk Two Moons)

The scene you are referring to in _Walk Two Moons_ occurs in Chapter 23, "The Badlands." When Sal’s mother says she wants to visit Idaho in order for her cousin, whom she has not seen in 15 years, to tell her what she is really like, she means that she wants to be told (or even reminded) what she was like before she was a mother and before she was married. It seems as if she wants to be reminded of the person she feels she no longer is. And, to come into contact with one whose last memory of her is of whom she used to be is why she wants to go to Idaho.