How to take screenshot of menu?

You have the option to capture the entire screen or the active window.

Entire screen

Install a tool for making screenshots, such as scrot, gnome-screenshot or ImageMagick, which provides import. Then run it with a delay from terminal, using one of these commands:

$ sleep 5; import -window root menu.png

$ scrot -cd 5 menu.png

$ gnome-screenshot -d 5

After running it, quickly open the menu and wait.

Active window

Both gnome-screenshot and scrot support taking a screenshot of the active window:

$ gnome-screenshot -wd 5

$ scrot -ucd 5 menu.png

Note that overlapping menus won't be compensated for but rather cut off: