My xfcebox hangs up regularly. Keyboard doesn't respond, mouse sometimes does. So, my question is: are there any ways (tools?) to determind what couses the problem, what log-files should I see etc.
You should check the following logs before anything:
Usually behaviour of the type that you described is not xfce's fault, but rather some lower-level operating system component, like your kernel or a kernel module.
Now I'm waiting for system to heng up to see these logs... ;D