I know how to check if a package is installed and if its available for install but I cannot find any list of packages that are installed with a release from Ubuntu.
Sometimes I am trying to answer something and I cannot remember if the tool I will refer was installed by default of if I installed in a later point and cant remember. What if its something I have and a user using another desktop environment does not?
I know I can recommend to install the package prior to asking him to use it but I would like to know if its installed by default or not.
Is there such list? Where can I find it?
Yes you can.
With each release, there is something called a "manifest" of the ISO created - this lists all the installed packages, their versions, etc.
Examples:
http://releases.ubuntu.com/hardy/ubuntu-8.04.4-desktop-i386.manifest
http://releases.ubuntu.com/lucid/ubuntu-10.04.4-desktop-amd64.manifest
http://releases.ubuntu.com/natty/ubuntu-11.04-desktop-i386.manifest
http://releases.ubuntu.com/oneiric/ubuntu-11.10-desktop-amd64.manifest
http://releases.ubuntu.com/precise/ubuntu-12.04-desktop-i386.manifest
http://releases.ubuntu.com/saucy/ubuntu-13.10-desktop-amd64.manifest
Newer releases are listed at http://releases.ubuntu.com/
Indirectly you could check Synaptic history of installation (In the File menu) type the name of your package you will know when it was installed or updated.