Relating specifically to the film, it would appear that she spotted that he was still moving. She then pretends to feel his pulse so that she has an excuse to linger above him, to better hear his response to her question.
NARCISSA: Is he alive...?
EXT. FORBIDDEN FOREST - CLEARING - NIGHT. Harry lies face down,
glasses askew. He OPENS his eyes a millimeter, finds Narcissa’s face
swimming in the dawn’s muted light, eyeing him with a strange
intensity: the question, he realizes, was posed to him.
BELLATRIX: My lord, let me help you -
VOLDEMORT: I do not require assistance.
Across the clearing, Voldemort rises shakily, Bellatrix at his elbow.
The Death Eaters watch, stirring uncertainly. Voldemort glances toward
Narcissa, toward Harry. Bellatrix detects the wary glint in his eyes.
BELLATRIX: The boy. Is he dead?
Harry’s eyes shift, meet Narcissa’s once again. Something in her expression... He closes his
eyes. She places her fingers over his heart,
NARCISSA [WHISPERS] Is he alive? Draco?
Harry hesitates and then... nods -- so subtly it’s barely perceptible.
Narcissa withdraws her hand, turns to where Bellatrix stands alongside
The Death Eaters CHEER. Hagrid HOWLS in misery. The news seems to
rejuvenate Voldemort. A fierce glint returns to his eyes. His stature
grows. He eyes the Elder Wand... and smiles.
Harry Potter and the Deathly Hallows, Part II - Original Screenplay