How do you tell the difference between Country music, Western music, and Folk music?

I realize that country música is ascendant now, but there is still a distinction between country and western music, and it seems to me that there's a great deal of overlap with certain kinds of folk music, such as bluegrass and many traditional folk ballads. What aspects of the música do you point to in order to identify a given piece as one or the other? It's always been hazy to me.