Background I've to take screenshots to demonstrate Conky‘s visual output.
I've chosen Byzanz after reading this answer on Ask Ubuntu.
Peek is more user-friendly, but I prefer CLI's precision. That's feasible thanks to a comment mentioning xwininfo.
After typing xwininfo, click on the target window. Switching to adjacent workplace is possible.
$ xwininfo xwininfo: Please select the window about which you would like information by clicking the mouse in that window.