Accompaniment Study 2: Root Notes

David, I've been away from playing for a while, have completed L0A2, but before progressing forward I am spending a little time here. So much great content, I want to enjoy the Journey rather than focus on moving on.

I've spent a little time in Accompaniment Study 2: Root Notes and playing this lesson. Helping me get a better base understanding of Root Notes and I know this will benefit me to spend more time here. 

Driving to work today I hear a song Cool Blues Walk by Eddy Clearwater and immediately I think that would be a great song to play this exercise to. At this point in my studies, is there any way to best identify what key the song is in? I spent some time in trial and error and certainly don't have a trained ear, but thinking this one may be played on a G Harp? Am I off?

Love the site, looking forward to spending more time here. Thank you.