Detect Visual Composer

if( defined( 'WPB_VC_VERSION' ) ) { ... }

works like a charm. I was searching for a possibility to hide the Nag screen that occurs when you get the WPBakery Page Builder as a bundeled plugin with a commercial theme to avoid customer's confusion. In case someone could need that snippet:

// hide nag screen of WP Bakery Visual composer if found
if( defined( 'WPB_VC_VERSION' ) ) {
    if(!isset($_COOKIE['vchideactivationmsg_vc11'])) {
        setcookie('vchideactivationmsg', '1', strtotime('+3 years'), '/');
        setcookie('vchideactivationmsg_vc11',  WPB_VC_VERSION, strtotime('+3 years'), '/');
     }
}

Hook that into admin_init and you're good to go!

only for WPBakery Page Builder

Actually, _wpb_vc_js_status since 4.8 is not correct, because it was not used anymore. The simplest way to check if page is using wpbakery - it is check for vc_row shortcode in content.

$post = get_post();
if ( $post && preg_match( '/vc_row/', $post->post_content ) ) {
    // Visual composer works on current page/post
}

Yes, you can detect if visual composer is enabled for a post. It's stored in the _wpb_vc_js_status post meta attribute.

$vc_enabled = get_post_meta($post_id, '_wpb_vc_js_status', true);

Note that a post can still contain the visual composer shortcodes, even when visual composer editing is not currently enabled. For example, if I setup a page with visual composer and then revert back to the normal editor, _wpb_vc_js_status will be false.

You can detect with is_plugin_active:

if ( is_plugin_active( 'js_composer/js_composer.php' ) ) {
     //your code here
}