Enable the Facelets Debug Page:
In web.xml you set Facelets to development mode.
WEB-INF/web.xml <context-param> <param-name>facelets.DEVELOPMENT</param-name> <param-value>true</param-value> </context-param>

The Facelets debug page gives you the line number in the .xhtml page where the error occured and access to the JSF component tree.

You can access the facelets debug page at any point by setting a hot key for that page.  To launch a debug popup on a page using CTRL-SHIFT-D, add <ui:debug hotkey=”d”/> in the page.

Enable the Seam Debug Page:
You need to add Facelets and Seam UI Support, then enable debugging in components.xml

WEB-INF/components.xml <core:init jndi-pattern="example/#{ejbName}/local" debug=true/>where example is the name of the application.

You can always access the seam debug page by going to the /debug.seam page in your browser.