Is it possible to detect if a POST action is for a “Save Post” before the save_post action?

Take a look at the Plugin API/Action Reference: Actions Run During an Admin Page Request..

Then add something simple to the appropriate hook:

function wpse19260_inspect_post()
{
    echo '<pre>';
        // print_r( $_POST );
        echo $_POST->ID;
        echo $_POST->post_content;
        echo $_POST->post_title
    echo '<pre>';
}
add_action( 'publish_post', 'wpse19260_inspect_post' );