Serial Number and Age

[airkingII]

Where do you find the watch's serial number - on/inside the watch? on the green tag?

Also I recall reading that you can tell how "old" a watch is or what model year it might be from letters in the serial number - any info on this would be helpful as I don't want to buy a Rolex that has been sitting in an AD's display case/back room for several years - at least if I know that it is "old" I could use that as leverage in getting a better deal.

Thanks