Use Online Guitar Tuners For Alternative Tunings.
Most guitar pieces are written in what is known as standard tuning: E, A, D, G, B, and E, low strings to high strings.
And most standard guitar tuners are designed to work in this format.
There are times however when an artist or composer pulls out a trick card and writes a very cool guitar piece in an alternative tuning.
A couple of guys who come to mind right away are John Mayer and Eddie Van Halen.
Nevertheless, there are methods to tuning your guitar to alternative tunings by using an online guitar tuner and actually listening to each note by ear.