Normally in Ubuntu if a new release is available the GUI will tell you. However, if you're running Ubuntu on the server, there is no GUI.
You can run
lsb_release -a, which will at least tell you what your current release is, but that doesn't tell you whether there is a new release you can upgrade to. I'm trying to write a dead-simple "determine whether it's upgrade time" script, and it would be really handy to be able to do so via a command ... does anyone know one?