There's a slight clue to Eva's feelings when we're told that she feels that Adrian's public manner has hardened toward her slightly. This is just after Adrian has finished his deck tennis match with the comely young Betsy D'Amido. Adrian goes to get a drink at the bar, and as Eva's feeling a tad dizzy, he agrees to bring her back a drink. It's then that she reveals something about her feelings:
You'll come back?
Note the hint of anxiety in her voice. She senses that something's up between Adrian and Betsy. When Eva drinks the cocktail that Adrian's brought back for her, she instantly feels better. But not for long. For later on, it becomes patently obvious to her that Adrian's been cheating on her with Betsy. How do we know? Well, later on in the evening Eva notices that Adrian has disappeared, presumably with Betsy. Despite her mind being addled with illness and champagne, Eva experiences a moment of clarity about her martial situation:
She had never tried to bind Adrian, never needed to—for they were serious people, with all sorts of mutual interests, and satisfied with each other—but this was a breach of the contract, this was cruel. How could he think that she didn't know?
Eva's no fool; she knows what's been going on between Adrian and Betsy. And a little while later, she openly confronts her husband about it:
"Eva, we'd better turn in."
Her lip curled. "So that you can leave me there and then come back to your eighteen-year—"
The truth is out. Adrian's been cheating on Eva with Betsy D'Amido, and it seems that the Smiths' seven-year marriage is coming to an end.