In The Last Wish shortly after leaving Nivellen Geralt pulls up to a stream to let Roach have a drink. Whilst drinking Roach backs away from the stream at the sight of "an ordinary devil's ring".
About midday, while Roach was drinking at a spring, the mare neighed piercingly and backed away, baring her yellow teeth and chewing her bit. Geralt calmed her with the Sign. Then he noticed a regular ring formed by the caps of reddish mushrooms peering from the moss.
“You're becoming a real hysteric, Roach,” he said. “This is just an ordinary devil's ring. What's the fuss?”
The mare snorted, turning her head toward him. The witcher rubbed his forehead, frowned and grew thoughtful. Then he leapt into the saddle, turned the horse around and started back, following his own tracks.
“Animals like me,” he muttered. “Sorry, Roach. It turns out you've got more brains than me!”
The Last Wish, "A Grain of Truth" IV
Geralt recites "Animals like me" after what Nivellen had told him shortly before leaving and turns back to go to the mansion.
“Animals like me,” boasted the monster. “And I like them, too. My cat, Glutton, ran away at the beginning but she came back later. For a long time, she was the only living creature who kept me company in my misfortune. Vereena, too—” He broke off with a grimace.
Geralt smiled. “Does she like cats too?”
“Birds.” Nivellen bared his teeth. “I gave myself away, pox on it. But what's the harm. She isn't another merchant's daughter, Geralt, or another attempt to find a grain of truth in old folk tales. It's serious. We love each other. If you laugh, I’ll sock you one.” Geralt didn't laugh. “You know your Vereena,” he said, “is probably a rusalka?”
The Last Wish, "A Grain of Truth" II
This all connects together when Geralt confronts Vereena and works out she is a bruxa, a type of vampire.
“You're so like a rusalka,” the witcher continued calmly, “that you could deceive anyone. All the more as you're a rare bird, black-haired one. But horses are never mistaken. They recognize creatures like you instinctively and perfectly. What are you? I think you're a moola, or an alpor. An ordinary vampire couldn't come out in the sun.”
The witcher backed away and clasped the hilt of his sword tighter. “That means you're—”
The corners of the lips started to turn up higher and higher; the lips flew open…
“A bruxa!” the witcher shouted, throwing himself toward the fountain.
The Last Wish, "A Grain of Truth" V
This all makes some sort of sense in connecting together but I can't really fit all the pieces together.
Why did Roach's reaction to the "ordinary devil's ring" make Geralt realise Vereena wasn't a rusalka?
What does "animals like me" have to do with it?
The connection with birds seems to be that as a vampire she has a bat form but I'm not 100% sure.
In short, what made Geralt realise that Vereena wasn't a rusulka but actually a vampire?