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 Casio watches (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 you have 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 this list you need to use the Casio module code that is contained within an etched 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.