One: one the plastic bag it came in (in the box)
two: Under the hand-guards. It is stamped on the barrel
Does yours have an M16 bolt carrier?
If so, it is likely it was made in or after march 2005.
That would be consistent with what we have seen around here
edit to add: a third option is to call Colt tomorrow and ask them (Customer Service) They will tell you on the phone with only the serial number