$  ssh -v localhost -s sftp
...
debug1: Authentication succeeded (publickey).
debug1: channel 0: new [client-session]
debug1: Requesting no-more-sessions@openssh.com
debug1: Entering interactive session.
debug1: Sending environment.
debug1: Sending env LC_MESSAGES = en_AG.utf8
debug1: Sending env LANG = en_US.UTF-8
debug1: Sending subsystem: sftp