So after some research I fount a way …
Stop the SASL daemon and start it by hand:
saslauthd -d -a pam -r -c -m /var/spool/postfix/var/run/saslauthd
OPTIONS settings in /etc/defaults/saslauthd for which options to use in your case.
But the most important option is
-d. It will run the daemon in the foreground and make it more verbose.
Now it will show you everything it does. 😀
Don’t forget to start the actual daemon once you are done debugging: