How can they tell that Old Dan and Little Anne have been walking circles around the big gum tree?

From the text:

Mr. Benson led the way. "As soon as we get out of this cane," he said, "we may be able to hear them. They have the coon treed in a big black gum tree. You're going to see a sight. Now I mean a sight. They've walked a ring around that tree clear down through the ice and snow. You can see the bare ground."