There are so many ways of telling your girlfriend, that she looks beautiful: you are beautiful, every time i see you, you look more beautiful, tonight you look so pretty. And, believe me, you should use all of them at least a dozen of times! However, sometimes, you just need to say something sweet, to show that you care, to notice that she is upset or happy – to do all of that with one sentence – it’s not an easy task!
Tell her, this new dress looks so good on you and enjoy her warming smile… Considering that the dress is really new (and you noticed it!). Otherwise, well… I wish you more luck next time.