How would you interpret the seventh sentence?
How would you interpret the seventh sentence “It would become you better to ask how you are to know it”?
Thank you.

PS: the excerpt is taken from At The Back of The North Wind written by the Scottish writer George MacDonald.
the context:
He sat there gazing out at a bed of tulips, which, although they had closed for the night, could not go quite asleep for the wind that kept waving them about. All at once he saw a great humble-bee fly out of one of the tulips.
"There! that is something done," said a voice -- a gentle, merry, childish voice, but so tiny. "At last it was. I thought he would have had to stay there all night, poor fellow! I did."
Diamond could not tell whether the voice was near or far away, it was so small and yet so clear. He had never seen a fairy, but he had heard of such, and he began to look all about for one. And there was the tiniest creature sliding down the stem of the tulip!
"Are you the fairy that herds the bees?" he asked, going out of the summer-house, and down on his knees on the green shore of the tulip-bed.
"I'm not a fairy," answered the little creature.
"How do you know that?"
"It would become you better to ask how you are to know it."
"You've just told me."
"Yes. But what's the use of knowing a thing only because you're told it?"
"Well, how am I to know you are not a fairy? You do look very like one."
"In the first place, fairies are much bigger than you see me."
"Oh!" said Diamond reflectively; "I thought they were very little."

Question

How would you interpret the seventh sentence?
How would you interpret the seventh sentence “It would become you better to ask how you are to know it”?
Thank you.

PS: the excerpt is taken from At The Back of The North Wind written by the Scottish writer George MacDonald.
the context:
He sat there gazing out at a bed of tulips, which, although they had closed for the night, could not go quite asleep for the wind that kept waving them about. All at once he saw a great humble-bee fly out of one of the tulips.
"There! that is something done," said a voice -- a gentle, merry, childish voice, but so tiny. "At last it was. I thought he would have had to stay there all night, poor fellow! I did."
Diamond could not tell whether the voice was near or far away, it was so small and yet so clear. He had never seen a fairy, but he had heard of such, and he began to look all about for one. And there was the tiniest creature sliding down the stem of the tulip!
"Are you the fairy that herds the bees?" he asked, going out of the summer-house, and down on his knees on the green shore of the tulip-bed.
"I'm not a fairy," answered the little creature.
"How do you know that?"
"It would become you better to ask how you are to know it."
"You've just told me."
"Yes. But what's the use of knowing a thing only because you're told it?"
 "Well, how am I to know you are not a fairy? You do look very like one."
 "In the first place, fairies are much bigger than you see me."
"Oh!" said Diamond reflectively; "I thought they were very little."

Answer

I'd understand it as something like this:  'It would be more appropriate to ask how YOU might know that".

'Become' as a transitive verb has the meaning of being suitable or fitting for someone.  [A relic of this meaning is the verbal adjective 'becoming']

The emphasised 'you' is important.   The tiny creature is throwing Diamond's question back at him and saying 'That's the question that you ought to be asking yourself' or in other words 'What makes YOU think that I must be a fairy?'. She's telling him that he shouldn't jump to conclusions.

Answer

I would interpret this as "It would be better for you if you asked how you can know it".

Answer

The language you have given is older English, or may be used today in a poetic way.  With "become you" or "becomes you" the word "become" is a different meaning than how it is normally used.  "I became tired" means "I changed from not being tired to being tired".  But, "become you" means something came to you and changed you.  "it would become you better" means "it would change you to be better".  There are more words that are like "become" which start with "be-".  Examples: "beset", "befuddle", "bedazzled", "beget".  The "be-" is used when something has been changed by something else.

Answer

Oh dear, not the easiest literature to read. 
"It would become you better...." means something like: it would do you good to / it would make you a better person....

"....to ask how you are to know it" is not common English, but means something like: to ask how you know this / to ask why you think you know this / to ask how you could understand this.

And the answer to this question is in the fairy's answer: "Yes, but what is the use of knowing a thing only because you are told it? (the answer)"  What is the benefit of knowing an answer if you don't understand what you are told  / if you don't understand the whole situation? 
It is like saying "you don't understand what you are asking" or 'you don't /won't understand it all, even if I explain it to you'.