What behaviors by the Ewells make the reader doubt their testimony?

It's fair to say that the Ewell family in To Kill A Mockingbird is widely loathed throughout Macomb. They're generally regarded as "white trash" especially by the self-appointed "respectable folk" of the town. Aunt Alexandra is particularly scathing, citing them as an example of her theory about bad character traits being passed down through families over generations. Even the normally equable Atticus hasn't much time for them, calling them "a disgrace to Maycomb for three generations."
So when Bob, no-good patriarch of the Ewell clan, accuses Tom Robinson of raping his daughter, we're immediately suspicious. On the witness stand, he loses no time in making a complete fool of himself, showing all the world just what a vile excuse for a human being he really is. Not only is it clear that he couldn't care less about the fate of Tom, he shows no true affection for his own daughter. When asked if he's Mayella's father, his response says it all:

"Well, if I ain't I can't do nothing about it now, her ma's dead."

His overall demeanor on the witness stand is arrogant, ignorant, evasive and smug. This doesn't by itself prove that he's a liar, of course, but at the very least it raises suspicions.
Mayella, for her part, gives a testimony that is full of holes. She changes her story and gives flatly contradictory answers. Finally, when Atticus asks Tom to stand up everyone can clearly see his withered left arm and shriveled hand. How then could he have beaten Mayella like she claimed he did?