The lens has a code stamped on the rear, the first letter indicates the factory
U = Utsunomiya, Japan
F = Fukushima, Japan
O = Oita, Japan
The second letter, is a year code that indicates the year of manufacture. Canon increments this letter each year starting with A in 1986 and prior to that, A in 1960 without the leading factory code. Here is a table to make things simple:
A = 2012, 1986, 1960
B = 1987, 1961
C = 1988, 1962
D = 1989, 1963
E = 1990, 1964
F = 1991, 1965
G = 1992, 1966
H = 1993, 1967
I = 1994, 1968
J = 1995, 1969
K = 1996, 1970
L = 1997, 1971
M = 1998, 1972
N = 1999, 1973
O = 2000, 1974
P = 2001, 1975
Q = 2002, 1976
R = 2003, 1977
S = 2004, 1978
T = 2005, 1979
U = 2006, 1980
V = 2007, 1981
W = 2008, 1982
X = 2009, 1983
Y = 2010, 1984
Z = 2011, 1985
My 17-40 L has a code UU0407 which is 2006, sounds about right for when I purchased it.
My 200 f/2.8 L has the code UA0627 - which I'm assuming is this year (as it's a new lens and I had to wait for it to come in to stock).
Links:
http://www.the-digital-picture.com/Canon-Lenses/Canon-Lens-Aging.aspx - this link might help if the rear code is missing.
http://bobatkins.com/photography/eosfaq/DATECODE.HTM