There are a few ways in which the book describes Tom Robinson. First, when Atticus introduces what he is about to do, to defend Tom Robinson, he says to Scout that Tom is a good man. He mentions that he is a "clean living man." Moreover, he is a member of Calpurnia's church. Most importantly, Calpurnia vouches for him and his family. Here is what he says:
“I’m simply defending a Negro—his name’s Tom Robinson. He lives in that little settlement beyond the town dump. He’s a member of Calpurnia’s church, and Cal knows his family well. She says they’re clean-living folks.
Arguably the most important description of Tom Robinson comes in the trial when Atticus shows that Tom Robison could not have beaten Mayella, because his arm was injured as a young man. In addition, the wounds on Mayella's face were consistent with the beatings given by a man who was a lefty, namely, Bob Ewell.
Here is what the text says about Tom Robinson in the courtroom:
His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I could see that it was no use to him.
“Scout,” breathed Jem. “Scout, look! Reverend, he’s crippled!”