Diagnosing power cycle vs software reboot

[Nikhil Nair]

Hi,

I'm running Ubuntu 14.04.5 LTS on a non-X server (i.e. remote access via
SSH).

"last reboot" indicates my server has rebooted 4 times in the last day and
a bit, none of which were expected.  I'd like to be as sure as possible
that these were power cut issues rather than a software issue, before
talking to my co-location provider.

I did an extra (deliberate) reboot, to see if I could tell the difference
somehow; I simply used "sudo reboot".  Output of "last reboot" after this
seems to indicate the first four were power issues (the last one is the
deliberate reboot):

(Command run at 10:09.)

reboot   system boot  3.13.0-100-gener Fri Nov 11 10:00 - 10:09  (00:09)
reboot   system boot  3.13.0-100-gener Fri Nov 11 09:06 - 09:59  (00:52)
reboot   system boot  3.13.0-100-gener Fri Nov 11 00:52 - 09:59  (09:06)
reboot   system boot  3.13.0-100-gener Thu Nov 10 19:45 - 09:59  (14:14)
reboot   system boot  3.13.0-100-gener Thu Nov 10 07:01 - 09:59 (1+02:58)
wtmp begins Tue Nov  1 09:02:54 2016

I'm not used to looking at output from this, but I notice the end time of
the first four is the time of the deliberate reboot.

Is there a recommended way of telling the difference, after the fact,
between a hard power cycle and a software reboot?  I've looked through
/var/log, but haven't found any obvious log lines that are created after
the reboot command and before the actual reboot.

Any help much appreciated!

Thanks, and best wishes,

Nikhil.