Watch the Keith Richards online guitar lesson by Jeff McErlain from Kings of Tone
Once you get used to the sound of open G you will be able to spot it very easily. As we know open strings have a great ringing quality that you really can't get with barre chords. A cool trick is what is called a cross tuning or mixed tuning. One I like is D,G,D,G,B,E from low to high. It is my sneaking suspicion Billy Gibbons uses the tuning on "Jesus Just Left Chicago" on the Tres Hombres record.