finding process id of a window

Hi,

I have few  X applications, started from the command line with identical
command line, but each has a different DISPLAY environment variable.
Is there a way to get the process id of a process from its window ?

Using xlsclients I can get the command line of the application, but since
all of them use same command line, I can't find unique process id.

Thanks for your help.

Yossi