Você pode usar qualquer um desses comandos,
apt-cache search <package_name>
aptitude search <package_name>
apt-cache policy <package_name>
sudo apt-get install <package_name> -s
Saída:
karthick@Ubuntu-desktop:~$ sudo apt-get install sun-java6-jdk -s
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package sun-java6-jdk is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package sun-java6-jdk has no installation candidate