Do you find the sergeant-major's story believable? Why or why not?
The story told to the White family by the sergeant major is clearly true, as is repeatedly emphasised by his tone and manner in describing it. Also his reaction to Mr. White's overtures to purchase the monkey's paw give his tale extra credence.
From the first mention of the monkey's paw, it is clear that this is an uncomfortable subject for the sergeant major - he tries to divert Mr. White from bringing up this tale:
"Nothing," said the soldier, hastily. "Leastaways nothing worth hearing."
His hastiness in trying to cut off this topic of conversation clearly indicates his hesitation in telling the tale, yet at the same time we can see that he is attracted by a captive audience and part-relishes his narration. Repeated throughout the first part of this story are numerous indications of his sincerity, despite the supposed ludicrous nature of his tale:
His manner was so impressive that his heared were conscious that their light laughter jarred somewhat.
His tones were so grave that a hush fell on the group.
In addition, his physical response to Mr. White's question about if he had used his 3 wishes also establishes veracity:
The soldier regarded him in the way that middle age is wont to regard presumptious youth. "I have," he said, quietly, and his blotchy face whitened.
Clearly, the author of this tale through all of these strategies presents the story of the sergeant major as being true and honest, based on his words and manner of describing the tale.