Matilda

Matilda comes to the realization that Miss Honey is poor from her bathing routine.

"I don't take a bath," Miss Honey said. "I wash standing up. I get a bucketful of water and I heat it on this little stove and I strip and wash myself all over."
"Do you honestly do that?" Matilda asked.
"Of course I do," Miss Honey said. "Every poor person in England used to wash that way until not so very long ago. And they didn't have a Primus. They had to heat the water over the fire in the hearth."
"Are you poor, Miss Honey?"
"Yes," Miss Honey said. "Very. It's a good little stove, isn't it?"

Matilda's second hint is Miss Honey's use of margarine, and the fact Miss Honey sais she never used sugar.

Margarine, Matilda thought. She really must be poor.