As Bob says Belarc Advisor will find all your old serial numbers and it is free. There is an equivalent for Macs but that costs $20. I came across a different method a while back that allows for a serial to be retrieved from a folder on your hard disk. I am not using any of the old products now , just Creative Cloud, where this method does reveal a serial number-at least under Windows 10, but can't test the older products
It works this way. You need to navigate to
'C:\ProgramData\regid.1986-12.com.adobe'
This is a hidden folder and so you need to first go into Control Panel and change your 'folder options' to make them visible. Once visible you should see files inside with *.swidtag extensions-these are standard xml files. Copy these files to the desktop and change the suffix to *.txt and open in a text editor. Towards the bottom you should see a field
<swid:serial_number>Your Serial Number here</swid:serial_number>
This is pretty academic for Creative Cloud as it uses a subscription model, yet a serial is still used behind the scenes. The thing I read said it also worked for CS3 and onwards so it may work with CS6, but no guarantees.