As many as one and a half million Americans became part of the Army Air Forces Ground Observation Corps. The Aeroplane Spotter was published in January 1940. By July of that year, months before the United States joined the war, the town of Mayfield, New York, had already organized volunteers and set up regular schedules on December 10, 1941.
The citizens of Kent, Connecticut, beat Mayfield to it, however, beginning the earliest Corp. They quickly sprang up and down both coasts. They chose the highest point in the area for their observation posts, of course, and usually worked in pairs. Citizens of every age took part, from high school students to senior citizens.
For all the preparation, only one German airplane ever entered American air space during the war. American airmen flew a captured plane to Florida. The locals weren't advised of the arrival, to test their effectiveness. They identified the make and model of the plane before it reached land.
Friendly aircrafts provided some amusement. A plane flew at treetop level in Connecticut, rising just in time to avoid the local hill. (The pilot in my book wasn't so lucky.)
While plane spotting continues to be a popular hobby (I found several journals for plane spotters at Amazon), the need for the organized corp carried over from World War II to the Cold War before dwindling away.