Configuration is in /etc/default/saslauthd.
testsaslauthd -u username -p password
This should be a system username, rather than a dovecot virtual username, if Dovecot is configured for virtual users.
Despite the existence of /var/spool/postfix/var/run/saslauthd if Postfix is installed, and a comment in the config file suggesting that OPTIONS can be set to use it, it actually is not happy with this, and will return this message:
connect() : No such file or directory
...if the Postfix version is used, instead of...
0: OK "Success."
...if the regular /var/run/saslauthd file is used.