Kaufman in "Advanced Birding" says that the most consistent character at
all seasons is the bill color: in Western (dull greenish yellow); in Clark's
(bright orange-yellow). He also adds that we consider the light in which we
view them. You might want to find a paperback copy of his book which appeared
in 1990. I've had trouble, when viewing a large grebe at a distance, trying
to see not only the bill color but if the white on the face goes above the
eye! Good luck--

Murray Hansen
Graham, WA
MurrayH at aol.com