And how do we know if Debian Linux kernel is as free as the Trisquel kernel
Inspecting the source code. But you can take the word of the FSF that made this inspection:
Previous releases of Debian included nonfree blobs with Linux, the kernel. With the release of Debian 6.0 (“squeeze”) in February 2011, these blobs have been moved out of the main distribution to separate packages in the nonfree repository.
https://www.gnu.org/distros/common-distros.html
