just make a PhaseListener and add it to your faces-config.xml. You get notified on Submit (Phase RESTORE_VIEW) and you could take the time until render (before RENDER_RESPONSE). Then you can use the time to log it to sysout. After rendering is complete you can't modify the output or add to the output.
if you want to quickly check the concrete request on client side could also use log - http://richfaces-showcase.appspot.com/richfaces/component-sample.jsf?demo=log&skin=blueSky
Just define it as popup and call whenerwer you need to monitor next request.
Thank you Guys