I only play by ear. I don't usually know what key I'm playing in. I know it's usually the first or dominate cord, so if some asks I can figure it out. My dad used to sing and play in a band and would keep "the circle of fifths" handy so he would know what cords to expect for a given key.
So, you up for the challenge ? You've already got the first part right for this particular song I have in mind. Merle Haggard's "I'm a lonesome fugitive" You can use the internet all you want.
