Alternatively, the instrument's serial number can be used to give a less accurate "rough estimate" of the age of the instrument. Check out our G&L Registry Page for a tabulation of instrument serial numbers that can assist in this regard (also be sure to add your G&L to the G&L Registry). Also check out the Chronology and Dating page on G&L's web site, which gives the start and end serial numbers for each year [up thru 1992].
The problem with these techniques is that G&L's serial numbers are non-sequential. The story I've heard on this is that they'll make up a whole series of bridges (and/or neck plates) and throw them into a bin, and then pick randomly from that bin when building guitars. As an example of the resultant scattering of serial numbers, consider that #G000519 (in theory, the 20th production guitar if the numbers were in order) is actually a late-1982 S-500!!
Beginning in February 2011, G&L went to a new serial numbering and serial number application. Beginning with serial number CLF061000, all serial numbering will be in sequence and the serial number is located on a metal plate attached on the back of the headstock. Also, a custom Specification Sheet detailing each instrument is included in the case. Note: Some Special Build and Limited Edition instruments can still be found with the previous water slide serial numbering.
In September 2016, the metal serial number plate was replaced with the waterslide decal on the back of the headstock.
Starting in February 2017, serial numbers will be laser-etched on the back of the headstock.
The serial number format will now be: "CLFYYMMnnn", where YY=last two digits of year, MM=two digit month.
Here is an example with the Specifications sheet (partial):

To get the completion date, contact Memo Romero [G&L Service Manager] and ask him to look up the serial number in the factory logs.
His email address is: repairs@bbesound.com.