You can login to your Raspi using ssh -CX pi@IPADDRESS and then run
DISPLAY=:0 scrot screenshot.png
to take a screenshot of the display that is currently attached. After that, use
feh screenshot.png
(due to ssh -CX this will display the image locally on your Linux desktop) or copyscreenshot.png to your local computer using scp, rsync, WinSCP or any other tool.
This is useful for debugging what is happening on your display.