On my debian machine something like journalctl -b 1 -k shows stuff. There’s also lots of debug files in /var/log/ like boot.logdebug, kern.log, messages, syslog.
But it somehow needs to be able to store the log on your disk. If the system craps out completely, it won’t get written to disk. The magic SysRequest keys might help if it only freezes. I learned “Raising elephants is so utterly boring.” You might wanna goggle that and learn how to do it.
Other than that, I mostly look at all logs (no ‘-b1’ and search for the place where it rebooted. Sometimes you find other related stuff while scrolling. But my own (old) thinkpad doesn’t ever crash.
I think there are other crash-dump tools available. It believe there’s something called ‘kdump-tools’ available on Debian. YMMV.