Help with hearing different tunings

[11:11]

So I've been watching this video (http://www.youtube.com/watch?v=tumzyV6RlY8&feature=related) and trying to figure out what tuning this guy is playing in, but i still can't figure it out. How do you guys tell what tuning someone is using? Guess and test?

[Twizx]

It seems like most acoustic sets these days are either half step down or full step down, so probably one of those.

[chrisstahl]

upon first listen it just sounds like standard to me. i am not near my guitar or a piano so i don't know if it is a half step down or anything but i see him goto an E major chord. He is just playing chords that allow the high B and high E string to be played open the entire time. I could be totally wrong. I know how to play but I don't know how to explain anything it is that I do. hope this helps.