Through much of the company`s history, Gretsch guitars were numbered sequentially and/or chronologically, so serial numbers (in addition to the features on the guitar) can give a very accurate idea of when the guitar was made.
That said, there are several tricks to successfully dating your Gretsch guitar.
Gretsch began numbering it's guitars sequentially in 1939, beginning with the better models.
This is particularly tricky in the mid `60s, when some serial numbers may belong to more than one numbering system.
One last thing to remember is that Gretsch years are similar to model years for cars.
With the advent of labels in the late '40s, the serial number is usually found on the label, which is usually visible through the soundhole on hollowbody models.
On solidbodies, it's usually inside the control cavity.