Look on the base and stem of your lamp. There should be a distinctive mark or markings somewhere on the lamp.
Check the lamp shade for any distinctive markings or words. The date of manufacture should be printed on the lamp on one of the places mentioned. If not, follow the steps below.
Think about all of the information you know about the lamp. Did you buy it new or second hand? Where did you buy it? Did it come from a family member? How long did they have it?
Go to your local antique dealer. Even though the Dale Tiffany lamps are not considered antique, the dealer is likely to know about the range of lamps and to be able to give you some information about yours.
Give them all the information you can about the lamp, and if you can, take it along with you to the dealer. They should be able to look over it with the information you've given them, and tell you the year that it was made.