When you have a program running in a hidden X screen (like with Xvfb for Selenium tests) you may want to look at that hidden screen occasionally.

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First, find out what X displays are currently active:

netstat -nlp | grep X11

This should give you some results like these:

unix  2      [ ACC ]     STREAM     LISTENING     8029600  4086/Xvfb           /tmp/.X11-unix/X99
unix  2      [ ACC ]     STREAM     LISTENING     8616     -                   /tmp/.X11-unix/X0
unix  2      [ ACC ]     STREAM     LISTENING     8615     -                   @/tmp/.X11-unix/X0
unix  2      [ ACC ]     STREAM     LISTENING     8029599  4086/Xvfb           @/tmp/.X11-unix/X99

In that case displays 0 and 99 are active and you can see that 99 is provided by Xvfb.

Next, simply make a screenshot from that display, e.g. with scrot:

DISPLAY=:99 scrot

This will create a timestamped PNG file in your current directory. You may also pass a custom filename to scrot.
Look at the screenshot with your favorite tool.