Narcissa (mother of Draco) is asked to check whether Harry is alive after he has been "killed". At this point Narcissa asks whether her son is alive. How could she trust Harry and believe him? Notwithstanding that she considered Harry to be her 'enemy'.
Harry could've have lied and said Draco was alive (if Draco was dead). In this case, Harry gets the benefit (as he escapes by being declared dead), while Narcissa is given the false hope that her son is alive. What matters isn't whether Draco is dead or alive, Harry could simply get away by saying yes.
So why did Narcissa trust Harry and take his word that Draco was alive?