To determine the year of your Sunfish, first look for the serial number on the outside of the transom, usually on the right just below the rubrail, and just embossed into the gelcoat. The last two numbers are the year.
If the serial number is not on the transom, either it has been fiberglassed over (which people almost never do), or the boat was built before 1973.
Sometimes the tag with the number is gone, but you can still narrow it down with the information about the changes in the hull and hardware with the information on this page
If all this fails, you might get lucky by looking through this Sailing Texas Sunfish photo gallery. Some color schemes were only offered one year. And if your gelcoat is original, you may be able to narrow the year of your boat to just a few years, if not one.