Bethshemesh was the first Israelite town they would come to, being on the border of Judah. (See the marginal reference.)
Related Commentaries of 1 Samuel 6:9
1 Samuel 6:9
9 And see, if it goeth up by the way of his own coast to Bethshemesh, 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.