SilentOne, yes, this is a very important character trait, and it is wonderful that you decided you want a girl that has this. I am sorry for the pain and suffering you went through in your first marriage. It is great that you can seek out this character trait as something important to you.
It is of course difficult to discern this on a date. But you can tell somewhat if the girl really has hakaras hatov if she thanks you for anything you do for her, whether it is opening a door for her, passing her a drink, giving her a tissue, etc. Also, you can try to do things that benefit her in some way and see if she acknowledges your actions and thanks you for them or compliments you on doing these things.