The view "action" of the module "content" uses the script action.php. You can find this out by inspecting the $ViewList variable in kernel/content/module.php. In the same array, there are mappings from POST variables to actions. In the view script (in this case action.php) the current action can be fetched by using $Module->currentAction() or there is checked if the current action is 'whatever' with $Module->isCurrentAction( 'whatever' ).
You can find some more information about this on http://ezpedia.org/wiki/en/ez/module
