Drupal API to check a particular user is logged-in, or not Is there an Drupal API to check whether a user is currently logged in or not, based on the user ID, such as is_user_login($uid)?
No, there is no function like is_user_login($uid), but you can see how the core block "Who's online" is created (source).
Drupal has a "sessions" table, where all current sessions are stored.