Use the shortcut "Ctrl+Shift+A" to search for an extension point. Definition and usage of an extension point use different icons. For example to see which plugin defines the extension point "org.eclipse.menu.ui" enter it in the dialog and select the entry with the blue icon.

Plugin Search allows you to perform a detailed search for extension points. Select the menu "Search" -> "Search" and the tab "Plug-in Search". The following will search for all declaration (defining an extension) to the extension point "org.eclipse.debug.core.launchConfigurationTypes".

Use the tab "Java Search" to search for Java elements, e.g. methods. To include all plugins from your target platform into the Java Search select the option "Include all plug-ins from target into Java Search" under the menu entry Windows-> Preferences ->"Plug-in Development".

The search show all references. You can double-click on a search entry to navigate to the coding.