“Stand at the crossroads and look.” What the people would look at was what they were doing and how that would turn out, and what they could be doing if they followed God’s way, and how that would turn out. The people of Judah were at a crossroads. They could listen to Jeremiah and repent and change their ways, or they could continue to live like they were living (which they did) and be destroyed as a result (which they were).
“the old paths.” This is most likely idiomatic for the ancient wisdom, “ask about the ancient wisdom.” Thus, “the good way” was the proper and godly way to live. The “ancient paths” were laid out in the Law of Moses. When people lived in accordance with the laws of God they found rest for their souls. There was no apparent value in simply walking on an old dirt pathway.