Determining the “current_page” in WordPress Custom Menus

for category:

  • li.current-category-ancestor
  • li.current-cat

for menu:

  • li.current-menu-item

for page:

  • li.current_page_item
  • li.current-page-ancestor

for post:

  • li.current-post-ancestor

For more details: http://codex.wordpress.org/Dynamic_Menu_Highlighting