If I understand correctly the information you are looking for is in the LinkType property of the PageData object. If you have the parent page you will need to load the PageData for each child in order to determine if it's an internal or external link.
EPiServer Development Team
Is there a way to determine whether a pageLink is internal or external by using the pageData.pageLink?
I have a page (parent) with subpages and want to determine the subpages link type to be able to treat them differently, can i do this by using the pageData.pageLink from the parent page?