What evidence is there that other men have camped there ?

From the text:

There is a path through the willows and among the sycamores, a path beaten hard by boys coming down from the ranches to swimin the deep pool, and beaten hard by tramps who come wearily
down from the highway in the evening to jungle-up near water. In front of the low horizontal limb of a giant sycamore there is an ash pile made by many fires; the limb is worn smooth by men who have sat on it.