Please refer the the man pages for useful tools such xprop, xclientlist, xwd

Also see http://www.xfree86.org/~mvojkovi/restest.c and http://www.freedesktop.org/Software/xrestop for examples of X-Resource extension usage. 'XRes' is a new XFree86 server extension (> 4.2) which can used to monitor client resource usage and detect server side pixmap leaks.