We see in the Deathly Hallows Ollivander examining wands taken from the Malfoy Manor. He identifies Bellatrix's, Draco's and Pettigrew's wands.
When he identifies Draco's wand, he senses that the wand changed it's allegiance to Harry. How can he tell that? Does the wand feel a certain way, does he listen to it?
How can you tell whether a wand has changed allegiance?
Relevant quotes from the book:
“Can you identify these?” Harry asked.
The wandmaker took the first of the wands and held it close to his faded eyes, rolling it between his knobble-knuckled fingers, flexing it slightly.
“Walnut and dragon heartstring,” he said. “Twelve-and-threequarter inches. Unyielding. This wand belonged to Bellatrix Lestrange.”
“And this one?”
Ollivander performed the same examination.
“Hawthorn and unicorn hair. Ten inches precisely. Reasonably springy. This was the wand of Draco Malfoy.”
“Was?” repeated Harry. “Isn’t it still his?”
-Harry Potter and the Deathly Hallows, Chapter Twenty-Four (The Wandmaker).