How can I tell if a post has been published at least once?

We can do this by storing the value into postmeta when post is published first time.

function save_ispublished( $post_id ) {

    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE)
        return;

    $published_once = get_post_meta( $post_id, 'is_published', true );

    // Check if 'is_published' meta value is empty.
    if ( ! empty( $published_once ) ) {

        $published_once="yes";
    }

    // store is_published value when first time published.
    update_post_meta( $post_id, 'is_published', $published_once );
}
add_action( 'save_post', 'save_ispublished' );

you can check it by get the meta value.

$is_published = get_post_meta( $post_id, 'is_published', true );

if( $is_published == 'yes' ) {

    /*
    * Actions if post is already published atleast once.
    */
}

Hope this help !