wininfo [ OPTION ] ...
X Window Information is a window information utility for developers of applications, toolkits, and window managers. X Window Information follows your pointer providing information about the windows below. Information presented includes:
A detailed description of the window hierarchy below the pointer.
Parsed interpretations of standard properties from both the application
window and the window manager.
Information about X server resources used by the application.
X Window Information accepts the standard command line arguments of any Gtk+ application. Unfortunately, these seem to be undocumented.