Spy++ -like tools on linux

How to get the list of open windows from xserverFrom 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,strace64 for 64bit binaries

ltrace -Tracks runtime library calls in dynamically linked programs