Check if the post is bookmarked by user (user id or by current logged in user) https://codeboxr.com/documentation-for-cbx-bookmark-for-wordpress/

gistfile1.txt

//check if the current user has bookmarked any post
if(class_exists('CBXWPBookmarkHelper')){
    $current_user = wp_get_current_user();
		$user_id = $current_user->ID ;
    
    global $post;
    $post_id    = $post->ID; //if you are trying outside of      
    
    $bookmark_by_user = CBXWPBookmarkHelper::isBookmarkedUser($post_id, $user_id);  //true or false
}