The command dmesg will show kernel messages. There's a limited amount of messages which are kept, and this shows the last messages. If you run it directly after boot you should see all the messages since the system was started.
You can also look in the /var/log/messages file. This is not truncated between boots, so you might need to use the date and time to work out when the last boot happened. The first message I see on boot is something like this:
syslogd 1.4.1#18ubuntu6: restart.
...but it might be a little different on your system.