A lot of the time you will be able to find that in dmesg which is the log that is created when the system is enumerating the hardware during boot. Probably something like a message that firmware isn't found.
Of course, some piece of hardware that doesn't work is often a good indication too.
[edit] I guess I should also mention the Debian Wiki in case you aren't familiar with it..
https://wiki.debian.org/Firmware
There are some threads on this forum about the microcode fiasco, you might want to review them too just for fun.