I think you can thread it back to the singing breakman- Jimmy Rodgers in the 20s. Blues and Yodeling- Now that's a rockin' combination! then thars all that rockabilly. Dang, thats good stuff. Rauchier than the touring swing bands from the 30s and 40s- When the white country bands really integrated the blues. Rock came right out of jump blues. Memphis' Sun Studios in the 50s- Carl Perkins was a groundbreaker.
But I would say a defining criteria would be the lead electric guitar and strong, driving rhythm section. And some obvious country/folk element- yodeling optional.
When country music is good, it is very very good. When it's bad, it's horrid.
Joke:
Q: How many female singers does it take to sing a Patsy Cline tune?
A: Apparently all of them.
Ok, this just in. Comming attractions (or repulsions?) Country-Bluegrass
Pink Floyd. I admit its intriguing.