But what is the evidence in the book saying the white mean were Wallace's?

Text Evidence:

"Finally T.J. said, “Okay. See, them Berrys’ burnin’ wasn’t no accident. Some white men took a match to ’em.”

TJ's explanation for how he knew:

"He smiled smugly. “’Cause your mama come down on her way to school and talked to my mama ’bout it.”