I'm assuming you mean folk music. In that case, folk music usually have simple melodic structures that are repetitive and if they have words to it, usually have some sort of historical significance. However if you want to get all music theory about it, here are some sites that I found could be helpful because I'm a little rusty on my music theory knowledge:
http://music.stackexchange.com/questions/22367/what-characterizes-american-folk-music
http://study.com/academy/lesson/what-is-folk-music-definition-artists-history.html
https://en.wikipedia.org/wiki/Folk_music#Characteristics
But if you are trying to learn how to identify folk music when you hear, the key thing is to listen to as much of them as you can. Back when I took music theory, my teacher would have us listen to songs and would make us identify their era and genre (starting as early as folk and Renaissance music up to modern stuff). Sometimes you just get this gut feeling and you just know however that's only after you familiarize yourself with each of them.
