Albert Barnes Commentary 1 Samuel 6:9

"And see; if it goeth up by the way of its own border to Beth-shemesh, then he hath done us this great evil: but if not, then we shall know that it is not his hand that smote us; it was a chance that happened to us." — 1 Samuel 6:9 (ASV)

Bethshemesh was the first Israelite town they would come to, as it was on the border of Judah.