Check if a custom post type has already been created

To check for the existence of a post type, use post_type_exists. For instance like this:

if ( ! post_type_exists( 'yourpt' ) {
     $args = array(
         // your new post type's arguments
     );
     register_post_type( 'yourpt', $args )
}

As for the prefix, that’s a matter of personal taste and how common you think the post type otherwise is. Personally, I don’t think it’s necessary.