John Gill 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)

And he said, you have asked a hard thing
Not a common privilege, but what is rarely enjoyed, and difficult to obtain, few are so favoured of God:

nevertheless, if you see me when I am taken from you, it shall be so
to you ,
but if not, it shall not be so; meaning, that if his rapture was visible to Elisha, and he was favoured with a sight of his assumption, and be an eyewitness of it, this would be a token both to Elijah that it was agreeable to the Lord to ask of him this favour for him, and to Elisha to expect it, otherwise not.