apt-cache policy pkgname
is the most convenient command, I think, but does depend on the sources you have configured in your
A good summary page for package information is Debian Package Tracking System. You could also use Debian Packages, but I find the former more convenient and more comprehensive. The former contains information about backports and also links to the corresponding Ubuntu packages, along with other information which the latter lacks.
BBW, PPAs are an Ubuntu feature and do not exist for Debian.