Debian backports: How do I list all available upgrades from bullseye-backports

I've searched unsuccessfully for a *method* to **list** all of the upgrades available from Debian's backports (specifically bullseye-backports): * I've added deb http://deb.debian.org/debian bullseye-backports main contrib non-free to /etc/apt/sources.list. * I've run sudo apt update successfully * I've run sudo apt list --upgradeable bullseye-backports with an "empty" result * I've run sudo apt list --upgradeable -t bullseye-backports; got an error * I've run sudo apt -t bullseye-backports upgrade; I get a list of NEW packages to be installed, and a list of EXISTING packages to be upgraded, and a [Y/n] option to proceed with the entire lot. Is there no way to simply ***list*** the upgrade-able packages from bullseye-backports?