The Casio watch date tool
Casio serial number, batch or module code dating
Casio have not consistently used serial numbers on their watches so there are 3 possible options to help narrow down the date of your Casio watch.
Some high end Casios (including G-Shock models) have a serial number, its 6 characters long and on the back of the watch case. The second character can be an O, N or D but the rest are numbers.
Modern solar powered Casio watches may contain a batch code, its 8 characters and contains numbers and letters, the last character will always be a letter.
If your Casio has neither of these or you need to further narrow it down then your last option is the production dates for the watch module, for this I have collated the release dates of as many Casio modules as I could find (over 1600 so far). To search it you use the Casio module code that is contained within a box on the case back. It is between 2 and 4 digits long. You do not need to open the watch for any of these codes.