Or HaChaim on Genesis 42:15/אור החיים על בראשית מ״ב:ט״ו

1 בזאת תבחנו, "You will be tested as follows:" Joseph expressed readiness to put their words to the test by the yardstick they themselves had provided, although this was not sufficient proof for their claim that they had come to Egypt to obtain the release of their brother.

2Alternatively, Joseph meant that if their younger brother would confirm that all his brothers had come to Egypt to locate and free their lost brother, he, Joseph, would believe all they had said.