Read the book with close attention to the geographic clues the author gives about the setting. The location of the boys' home is a rugged wilderness at the foot of a mountain. The fact that the area is at risk of wildfires each summer suggests that the family probably lives somewhere in the western half of the United States.
Notice the information about the setting the author includes. The family lives on a farm -- pictures in the book depict the farm as modest and neat. Because Bosco Burak must "pack in" to Call Back Mountain with two mules, this suggests the area is remote without roads for car travel.
Find the two clues the author provides that give an indication of the time the story takes place. Bosco Burak brings CDs with him up to the fire tower and he listens to them with the boys. Because manufacturers did not release CD players for sale until the early 1980s, logic suggests that the story must take place sometime after this -- at the end of the 1980s or later. Another clue that suggests a modern setting is the helicopter that flies in to pick up the body of Bosco Burak after he dies of a heart attack at the fire tower.