Spy++ - like tools on linux

How to get the list of open windows from xserver

From the CLI you can use
xwininfo -tree -root
When coding, use the XQueryTree function from the Xlib library.

See also Does anyone know something like WinSpy on Xwin?

• xwininfo - For getting some window parameters(part of xorg install)
• wininfo - A small gtk gui application which shows the hierarchical
  information
• editres - Examine application configuration aka resources(also part of x)
• xmon - monitors the x protocol
  xev - X event viewer)
• xlsclients - list the clients of the X server
• lsof - List open files on the system (including non regular files such as sockets and open shared libraries)
• strace - system trace, traces system calls made by your program