Debian doesn’t log boot log messages by default.
To enable logging of boot messages in Debian, edit the /etc/default/bootlogd file, and change BOOTLOGD_ENABLE=No to BOOTLOGD_ENABLE=Yes . The messages will be logged at /var/log/boot (and browsable in the Log File Viewer)
To check messages during booting without logging you can pause the scrolling with Control+S (resuming with Control+Q). Go up and down using Shift+PgUp and Shift+PgDn.