How can I list default lpr filters?

This is not the most satisfying solution, but setting

LogLevel info

in /etc/cups/cupsd.conf (then restarting CUPS e.g. with sudo systemctl restart cups), the filters are listed in CUPS logs when a job is sent.

CUPS logs being handled by journald by default in Fedora (28, at least), they can be accessed by

$ journalctl -b -u cups
…
juil. 23 15:31:56 Schenker cupsd[14390]: [Job 20] Adding start banner page "none".
juil. 23 15:31:56 Schenker cupsd[14390]: [Job 20] Queued on "Brother_MFC-9330CDW" by "goug".
juil. 23 15:31:56 Schenker cupsd[14390]: REQUEST localhost - - "POST /printers/Brother_MFC-9330CDW HTTP/1.1" 200 358 Create-Job successful-ok
juil. 23 15:31:56 Schenker cupsd[14390]: [Job 20] File of type application/pdf queued by "goug".
juil. 23 15:31:56 Schenker cupsd[14390]: [Job 20] Adding end banner page "none".
juil. 23 15:31:56 Schenker cupsd[14390]: [Job 20] Started filter /usr/lib/cups/filter/pdftopdf (PID 14599)
juil. 23 15:31:56 Schenker cupsd[14390]: [Job 20] Started filter /usr/lib/cups/filter/pdftops (PID 14600)
juil. 23 15:31:56 Schenker cupsd[14390]: [Job 20] Started filter /usr/lib/cups/filter/brother_lpdwrapper_mfc9330cdw (PID 14601)
juil. 23 15:31:56 Schenker cupsd[14390]: [Job 20] Started backend /usr/lib/cups/backend/dnssd (PID 14602)
juil. 23 15:31:56 Schenker cupsd[14390]: REQUEST localhost - - "POST /printers/Brother_MFC-9330CDW HTTP/1.1" 200 40108 Send-Document successful-ok
juil. 23 15:31:59 Schenker cupsd[14390]: [Job 20] Job completed.
juil. 23 15:31:59 Schenker cupsd[14390]: Expiring subscriptions...

NB : Option -f of journalctl may proves handy.