How do we know that the house in which Helen Keller lived was beautiful?

We know that Helen considered the house where she grew up beautiful because of the gardens surrounding it.
Helen spent her early childhood in a little house on the Keller homestead.  In general, whether your question refers to that or the homestead in general, Helen’s childhood was full of beauty because she loved nature.

The Keller homestead, where the family lived, was a few steps from our little rose-bower. It was called "Ivy Green" because the house and the surrounding trees and fences were covered with beautiful English ivy. Its old-fashioned garden was the paradise of my childhood. (Ch. 1)

When she could see, she was enamored of the shadows and the beautiful flowers. Even after she lost her sight and hearing, nature was a refuge to her.  Nature was important to Helen Keller because you do not need to be able to see or hear in order to enjoy it.  You can still smell and feel it.

… I used to feel along the square stiff boxwood hedges, and, guided by the sense of smell, would find the first violets and lilies. There, too, after a fit of temper, I went to find comfort and to hide my hot face in the cool leaves and grass. … (Ch. 1)

Even after Helen Keller’s teacher Anne Sullivan came, she still enjoyed being out in nature.  They would have their lessons outside in the beautiful gardens, instead of in a classroom.  Helen loved those lessons, and felt everyone should learn that way.
When Helen was five years old, she moved from the little vine-covered house to the big house, but it made no difference to her because the beauty of nature was what mattered.  She could smell that beauty.  What the house actually looked like would not have mattered to someone who could not see and hear.