After a short period of overlap with the old system, the post-76 numbers will start with a letter that indicates the decade, followed by a number that indicates the year of that decade.
These dates will tell when the original part was manufactured, but are not exact indicators of when the guitar was actually put together and finished.
Here is what the neck date and body date look like from a 1952 Telecaster: If you're not comfortable removing the neck of a guitar to peek at the date marker, I encourage you to take it to a local tech or luthier.
I will also mention briefly pot-codes as a resource (numbers on the internal potentiometers of the guitar).
In this early period, the serial number can be found on the bridge of the instrument (see image).
Here are the rough serial number ranges for the early Esquires and Telecasters: By mid-1954, Fender began using a universal serial number sequence for all its instruments.
Starting in 1976, Fender transitioned to a new serial number scheme and moved the placement of most serial numbers to the headstock of the instrument.