Well, there wouldn’t be any direct book parts on how Dany is Cersei’s “younger and more beautiful” queen, because, well, Dany hasn’t even met Cersei yet. (I don’t think Dany even knows Cersei exists, and Cersei barely knows Dany exists and really doesn’t care about her.) Which I think is the basic proof there, I mean:
“Queen you shall be… until there comes another, younger and more beautiful, to cast you down and take all that you hold dear.”
Is Cersei cast down yet? Technically, yes, but she’s coming back. Has all that she holds dear been taken from her yet? No. Joffrey may be dead, but Tommen and Myrcella are still alive, so I don’t think Sansa’s hairnet really counts (which Sansa did not choose to use, Olenna did) — and Margaery also doesn’t count. And it’s not just her children that Cersei holds dear — it’s her power, her possessions, her beauty, and even Jaime.
Also, the simple fact that Cersei suspects the younger queen is Margaery (and probably suspected Sansa when she was betrothed to Joffrey) should be proof that it is neither of them. (Just like Cersei believes Tyrion to be the valonqar, when it’s Jaime.) Sansa is also not yet a queen, nor is it definite that she ever will be. (I think Sansa will be in a position of power by series end, but not necessarily a queen, nor will it likely happen before Cersei dies.) But Dany is a queen, younger than Cersei, and arguably far more beautiful — she’s even been called the most beautiful woman in the world. And she is far more capable of casting Cersei down and taking all she holds dear than any other queen or potential queen in the series.
But the real point is not who the queen actually is, it’s that Cersei’s efforts to avert the prophecy are what will bring her downfall. It’s like Greek tragedies, like Oedipus, the very definition of literary irony. For more of this, see this post, and this tag, I’m not saying anything new here I’m afraid.