Or HaChaim on Genesis 44:15/אור החיים על בראשית מ״ד:ט״ו

1 הלא ידעתם כי נחש ינחש איש אשר כמוני, "Did you not know that a man such as I is able to divine things?" With this comment Joseph wanted to answer the brothers' accusation that he had framed them, since how could he have known that they had committed the theft unless they had themselves confessed to it? Joseph headed off such an accusation by saying that someone like himself practiced sorcery.

2The reason the Torah has Joseph describe the sorcery as being performed not "by myself," but by someone "in a high position such as I" was, that not just he but anyone who occupied high office was presumed to be able to divine things like that. The brothers should have taken this into consideration before stealing anything.