Verse-by-Verse Bible Commentary
Adam Clarke Commentary - 1Sam. 14:9
If they say thus unto us - Jonathan had no doubt asked this as a sign from God; exactly as Eliezer the servant of Abraham did, Genesis 24:12.
Albert Barnes' Notes on the Whole Bible - 1 Sam. 14:10
Though it is not expressly said, as in the case of Gideon Judges 6:34, Othniel Judges 3:10, and others, that the Spirit of the Lord came upon him, yet the whole course of the narrative, especially 1 Samuel 14:13-16, indicates an extraordinary divine interposition.
Wesley's Explanatory Notes - 1Sam. 14:10
But if they say thus, Come up unto us; then we will go up: for the LORD hath delivered them into our hand: and this shall be a sign unto us.
A sign — Jonathan not being assured of the success of this exploit, desires a sign; and by the instinct of God's Spirit, pitches upon this. Divers such motions and extraordinary impulses there were among great and good men in ancient times. Observe; God has the governing of the hearts and tongues of all men, even of those that know him not, and serves his own purposes by them, tho' they mean not so, neither does their hearts think so.