In a terminal run
apt-cache policy with the name of the package. This will show you what version you have installed, if any together with what is available in the Ubuntu repositories. For example if on my machine I enter
apt-cache policy sudo, the output is:
*** 1.8.3p1-1ubuntu3.4 0
500 http://gb.archive.ubuntu.com/ubuntu/ precise-updates/main i386 Packages
500 http://security.ubuntu.com/ubuntu/ precise-security/main i386 Packages
500 http://gb.archive.ubuntu.com/ubuntu/ precise/main i386 Packages
Showing that I have version 1.8.3p1-1ubuntu3.4 installed and this is the latest.
In addition you can use the Ubuntu Packages Search website to see what is available searching by name or description. Though this will not tell you what you have installed.