Q. How do I know what packages are available to be updated on my Debian / Ubuntu Linux system?

A. You can use standard command line utility called apt-get or GUI tool called Synaptic Update-Manager.

List all packages which could be updated

To list outdated package use apt-get command has following two options:
=> -u : Show upgraded packages; Print out a list of all packages that are to be upgraded.
=> upgrade : It is used to install the newest versions of all packages currently installed on the system

Type the following command at shell prompt:
$ sudo apt-get upgrade
$ sudo apt-get -i upgrade
Output:

Reading package lists... Done
Building dependency tree... Done
The following packages will be upgraded:
  debconf debconf-i18n e2fslibs e2fsprogs file findutils klibc-utils libblkid1 libc6 libc6-i686 libcomerr2 libklibc libkrb53 libmagic1
  libpam-modules libpam-runtime libpam0g libss2 libssl0.9.8 libuuid1 linux-image-2.6-686 linux-image-2.6.18-5-686 locales perl perl-base
  perl-modules tar tzdata
28 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 39.6MB of archives.
After unpacking 148kB of additional disk space will be used.
Do you want to continue [Y/n]? 

To updated just say yes to prompt – “Do you want to continue”. Alternatively, you can type the following command:
$ sudo apt-get update
$ sudo apt-get upgrade

Synaptic GUI Package Update Notifier

Synaptic works hand-in-hand with Update-Manager to inform you about updates to the software installed on your system. If software or security updates are available, Update-Manager will display a pop-up message and an icon in the notification area on your GNOME desktop. To update your system, click on the notification icon, enter your password and click OK.

(Fig. 01: Synaptic GUI Package Update Notifier [ Image credit Ubuntu Project] )