Currently there is no way to find out (except from ugly and/or implementation-specific hacks) to find out if a user provided content for a <ui:insert name="xxx"/> or not.
Scenario: Imagine you want to provide an optional search form, which the user can define via <ui:define name="searchForm">. Then you can insert it via <ui:insert name="searchForm" />. However, what if you have to render some additional markup around the user provided search form, but you don't want to render it if the user did not provide a search form?
One very ugly workaround is to use a h:panelGroup, which you bind to a bean, use <ui:insert name="searchForm" /> as the only child and check via the binding if the h:panelGroup has any children.
It would be very nice to have an EL expression to check this stuff, or maybe to have a special facelets tag which only renders its content if a specific template is available, e.g. (note that I am not very good with names) <ui:ifTemplateAvailable name="">....<ui:insert name="xxx" /> ...</ui:...>
From the MyFaces-impl point of view, I think it is not very hard to implement.