No guessing required, the sun is a constant source and if he knows his materials the only thing that can change is what is between the sun and the subject.
Lots of photographers use the sunny 16 rule, or open shade rule

Quote Originally Posted by Lionel1972 View Post
While watching this interview of Steven Spielberg and Harrison Ford done after the 30th anniversary screening of "Raiders of the lost Ark", I got very surprised to hear them talk about how Douglas Slocombe, Director of Photography on the movie, never used any lightmeter but just "guessed" the right exposure. Fascinating and awe inspiring!