Nix: See all versions of package in the archive

Use apt-cache policy <package>. For example:

$ apt-cache policy nautilus
nautilus:
  Installed: 1:3.18.5-0ubuntu1~xenial1
  Candidate: 2017-03-03 05:34 1:3.18.5-0ubuntu1~xenial1
  Version table:
 *** 1:3.18.5-0ubuntu1~xenial1 500
        500 file:/media/Linux/apt-repo/xenial1  Packages
        500 http://ppa.launchpad.net/budgie-remix/ppa/ubuntu xenial/main amd64 Packages
        500 http://ppa.launchpad.net/gnome3-team/gnome3/ubuntu xenial/main amd64 Packages
        100 /var/lib/dpkg/status
     1:3.18.4.is.3.14.3-0ubuntu5 500
        500 http://us.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
     1:3.18.4.is.3.14.3-0ubuntu4 500
        500 file:/media/Linux/apt-repo/xenial1  Packages
        500 http://us.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
     1:3.14.2-0ubuntu13 500
        500 file:/media/Linux/apt-repo/wily  Packages

Via askubuntu.