Sorry, on the pi. Hopefully you can ssh into it and check the log files. From the documentation
This image has sshd on boot - that means you can immediately ssh in using raspberrypi.local! The ssh keys are generated on boot but since the user/pass is simply pi/raspberry you should not put this on an accessible network until you've changed the password.
I'd think you should be able to ssh in before you type startx and use "ls -lt /var/log" to identify whichever log file is getting written to while your startx loop is happening. Then you can "tail -f /var/log/whatever.log" to watch what's happening in realtime.