A command that is specifically intended for this is apt-show-versions. You often have to install it, but then can run apt-show-versions -a and it will show you the version number, the distribution (i.e. testing, stable, unstable, backports, etc.) where that can be found, and finally tell you if the version you have installed is up to date or not.
It does not give you as much information as apt-cache, but gives you pretty much what you need, as you can then install from the correct repository (using aptitude / apt-get -t) or simply install using the correct version number in the form you noted.