She asked him because if Draco was still alive, she had something to fight for. She had a reason to lie to Voldemort. If Draco is alive, she needs to get into the school to see him and protect him, and helping Harry feign death was the easiest way to do that. But if Draco was already dead, she wouldn't have bothered.
Harry revealed that Draco was still very much alive, and she lied to her master in order to get closer to him. Narcissa may have concealed the truth from Voldemort to save her son, but she inadvertently helped Harry to victory in doing so.
When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
Narcissa has eyes (sense of sight). Narcissa also has hands (sense of touch). She saw that Harry was still breathing. When she touched him she could feel him move.
When Voldemort "killed" Harry in the Forbidden Forest, she asked him whether Draco was still alive, as she knew that the only way she and Lucius could search for Draco inside Hogwarts would be as part of Voldemort's victorious army.
In a whisper, she asked if Draco was alive. When Harry replied quietly that he was, Narcissa betrayed the Dark Lord and asserted that Harry was dead. She knew the only way her family would be reunited was as part of Voldemort's conquering army, thus proving she loved her family more than she supported Voldemort.
Of course she did. She felt his heart beating, and it usually means the person is alive. Why didn't Narcissa Malfoy kill Harry after she found out Draco was okay? She'd have preferred to kill Voldemort, who punished and humiliated her family and tried to get her only child killed by setting him against Dumbledore.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
When Harry revealed that he didn't actually die in the Forbidden Forest, some Death Eaters could probably foresee that Voldemort didn't have long before her would get killed at Harry's hands and so left for a safe refuge so as to avoid prosecution at the hands of the Ministry of Magic.
Following his failure to obtain an important prophecy at the end of Order of the Phoenix, Lucius is sent to Azkaban for a while until Voldemort breaks him out, although he doesn't play much of a role in the series going forward.
In fact, Lucius' own support to Voldemort is revealed to come only under some form of duress, which is likely Narcissa's Veela Allure. When Lucius turns on Voldemort due to the reveal of his Horcruxes, Narcissa divorces him, taking 75% of Malfoy's wealth with her.
He was scared and uncomfortable. The death eaters were staying at their place. The family was deprived of their privacy. Lucius lost his wand when Harry's broke it!
Having rendered unknown, shady (and almost certainly magical) services to King William I, Malfoy was given a prime piece of land in Wiltshire, seized from local landowners, upon which his descendants have lived for ten consecutive centuries.
Narcissa the devoted mother
If Bellatrix managed to muster affection for anyone other than Voldemort, it was her younger sister (even at the height of the Second Wizarding War, Bellatrix and Narcissa called each other Cissy and Bella, no doubt childhood nicknames they'd given each other).
We know the Elder Wand allied with Draco Malfoy, who disarmed rather than killed Albus Dumbledore. It later switched its loyalty to Harry Potter during his escape from Malfoy Manor.
She was a good wife. She and Lucius seemed affectionate enough what with her comment to Harry about how she would be reunited with her darling husband [who had been residing in Azkaban] and seen giving him so small measure of comfort after Voldemort took his wand.
Due to his humiliation in the Triwizard Tournament, Cedric eventually becomes a Death Eater and kills Neville Longbottom. Due to Neville's death, the final Horcrux Nagini was never destroyed, and Voldemort is never defeated.
Peter Pettigrew, (a.k.a. Wormtail), is the only Death Eater known to have been in a House other than Slytherin (Gryffindor) while at Hogwarts.
Harry describes Umbridge to Sirius Black, saying that she's vile enough to be a Death Eater, despite not being one. Umbridge is also depicted to be a prejudiced person, despising half-breeds and Muggle-born witches and wizards to the point of firing Hagrid due to his half-giant birth status.
Avada Kedavra, also known as the Killing Curse, kills a person instantaneously and without injury. There is no countercurse for it, and only one person, Harry Potter, has ever survived it.
Phineas Nigellus's portrait inside it overheard Hermione and reported that to Snape. Snape thus knows that Harry and Hermione are in the Forest of Dean, though not exactly where; the Forest of Dean is a largish place, several wooded sections totaling about 40 square miles.
Avada Kedavra - Used 8 times.
It is common to see this curse used by the Death Eaters and by “He Who Must Not Be Named.” This spell is based on the Aramaic, meaning "disappear like this word."
Malfoy spots Harry and casts a Cruciatus curse; Harry, defending himself, using the Half-Blood Prince's Sectumsempra spell without knowing its effects. To his horror, it gashes Draco's face and chest, spilling his blood everywhere.
Just before Christmas, Harry overhears a conversation in the Three Broomsticks pub that Black betrayed James and Lily Potter to Voldemort, and afterward murdered their friend, Peter Pettigrew, along with twelve Muggle bystanders. Harry is shocked to learn that Black is also his godfather.
Why? The main reason is that Lucius failed to retrieve the prophecy from the Ministry of Magic, put that on top of all the other times Lucius failed, and it leads to him losing any clout he once carried within the Death Eaters. Lucius did that for his own benefit - he was trying to discredit Arthur weasley.