The U.S. Forest Service has created an online map that illustrates where fall leaves are changing color. The map's key runs from green (leaves haven't changed yet) to red (majority of leaves have changed color) to brown (leaves are "past peak"). It is based off of eyewitness accounts.
There are individual maps available for national forests and states, as well as one that represents the entire U.S. The Minnesota map is available here.
The U.S. forest service is also maintaining a phone hotline about fall colors at 1-800-354-4595.
Photo credit: VancityAllie