Genesis 42:14

That is it that I spake Joseph seems to say that their claim to be all the sons of one man is improbable and suspicious. If these suspicions are to be removed, their statements must be verified. Their statement was either the needless embroidery of a falsehood, or it was a detail of actual life that could easily be proved. Joseph's real object is to find out about Benjamin, whether he was alive, and well treated by his brothers. It is a delicate touch in the story, that he abstains from cross questioning them about the brother that "is not."