Albert Barnes Commentary 2 Kings 2:10

"And he said, Thou hast asked a hard thing: [nevertheless], if thou see me when I am taken from thee, it shall be so unto thee; but if not, it shall not be so." — 2 Kings 2:10 (ASV)

It would be better to omit the words “when I am,” which are not in the original. The sign was to be Elisha’s seeing the actual translation, which he did (2 Kings 2:12).