I have 4 CFBuilder licenses. I've bought a new computer and moving to it. CFBuilder on the new machine is complaining that the serial number's been used too many times, and needs to be deactivated. How can I see the serial number on the old machine to enter the correct one in the new machine?
I doubt that you can see it on the old machine. Typically software will not allow you to see the serial number with which it was registered. Hopefully you kept your serial numbers when you purchased the software.
It is not straightforward but for your information sake - If you deactivate the license (Help>Adobe ColdFusion Builder Deactivation...) on your old machines (permanently by selecting the checkbox in the deactivation window), you are in a state where your licenses are not used anywhere.