There are a few times in Chapters 5 and 6 that Catherine's innocence and genuineness (if that makes sense) really show:

"Perhaps Catherine was wrong in not demanding the cause of that gentle emotion - but she was not experienced enough in the finesse of love, or the duties of friendship, to know when delicate raillery was properly called for or when a confidence should be forced."

Isabella is longing for Catherine to ask her what she knows about love! And Catherine misses the big hint.

Then when Isabella gets to the Pump Room a few minutes earlier than Catherine, she complains that she has been waiting 'this age!' Catherine says, "Have you, indeed! -I am very sorry for it; but really I thought I was in very good time." Of course she was, she was right on time!

When Isabella exclaims that Catherine is just the sort of girl to be a 'great favorite' of men, Catherine blushes. There is a bit of her that would like to believe that, I am sure, but she is not the kind of girl to even think such a thing of herself on her own.

Then there is a bit of a turn - Catherine does express her own independent opinion. They are talking about the book Sir Charles Grandison and Isabella says it's unreadable. Catherine says, "It is not like Udolpho at all; but yet i think it is very entertaining."