All Seiko's should have a Serial number on the case back
The first number in the Serial is the last number in the year of production, and so if it is a new watch and you see an 8 it should be the year 2008
The second number in the Serial is the month the watch was made if you see a 4 it will be the month of April
most Seiko serial are 6 digits long and so a serial of 86XXXX would be a watch made in April of 2008
The key to the whole trick is knowing what movement is in side and older movement like the 6309 was made in the late 70's and 80's and so a watch with 86XXXX would be April 1988 and so on .
A note on months since only one numberis used for the months Oct / Nov / Dec being two numbers is noted by letter
if you see a Zero it is a letter O for October N= Nov D= Dec
8DXXXX would be December 2008 8OXXXX would be October 2008
It does help to know a little about movement production dates when dating watches on the street but if you have a computer handy you can always drop by these fine websites:
Seiko Watch Production Date Calculator
http://www.csce.uark.edu/~jgauch/tools/seiko.html
Seiko production date finder
http://speedtimerkollektion.com/images/ . finder.htm