How to see boot error messages

I noticed that the Linux kernel has been updated several times since I installed Debian Jessie and decided to try a new version to see if it would help with a few issues I've been having.

I installed a newer kernel from Debian Backports, but during the reboot I noticed an error message flash by- far too quickly to read.

I was sure I had found a way to read such messages again after the computer finished booting, but just couldn't recall it.

After spending a couple of hours on Google trying to rediscover the secret, I found it.

dmesg

I've put it in red and highlighted it so I don't forget it again.

Running dmesg showed me the error message I'd missed, highlighted in red.

firmware: failed to load radeon/mullins

My new kernel couldn't use the non-free driver firmware I had installed- installing firmware from Debian Backports fixed the problem.