How to diagnose sudden reboot?

I have a custom image, containing lot of non-standard packages. Unfortunately, there are unexpected automatic reboots, without any meaningful logmessage before.
Any suggestions, how to get more info about the reason for the reboots ?

Ideally by keeping a serial console connected, followed by using ramoops (see the ipq806x/ r7800 exploration thread) - if you're very lucky using netconsole might already catch the issue.