Why does Biddy believe Orlick may have hurt Mrs. Joe?

About one month later after the attack on Mrs. Joe, she tries to tell the others around her, who her attacker was. As she couldn't speak, she tries to make the others understand through drawing the letter 'T' on the slate over and over again. Finally Pip understands it to be a picture of a hammer.

However, it was Biddy who understood the mysterious sign first.

"Why, of course!" cried Biddy, with an exultant face. "Don't you see it's him!"

Orlick, without a doubt! She had lost his name, and could only signify him by his hammer.

This shows that Biddy is quick to understand things. She is alert and observant. The people only people that are likely to use a hammer are Joe, Pip and Orlick. She is very thoughtful and considers the facts carefully.

Her one thought that the attacker is Orlick shows her ability to judge people through their behaviour. She trusts young Pip and gentle Joe not to harm anybody.

The attack was a result of the quarrel between Mrs. Joe and Orlick in the morning on the day that it happened. Biddy is capable of looking at matters in a third eye. Thus, she comes to a conclusion that it should be Orlick who attacked Mrs. Joe.