Where to check the updated packages or new packages list in launchpad
Each upload to the Ubuntu archive is announced via a mailing list. They are also available via RSS.
These are of course very high volume. Even after the release, all uploads to the proposed and security archives are announced.
UbuntuUpdates.org is a third-party service that also tracks this sort of information, including for specific PPAs:
Also interesting is Launchpad's build farm. Here you can see what packages are currently building in both PPAs and the official archives.
If you have a PPA (Personal Package Archive) on your Software Sources then you need to run
Which update all of your packages. You should then be able to update via Synaptic or Software Center.