With the old XMPP connection it''s still possible to know if someone is online and what app he''s using with this combinations:
-from PC online/away
*Unfortunally at the moment it''s impossible to know if someone is online or offline from mobile*
*It''s not a chat client!*
No personal datas are collected using this application.
Sources are avaiable on github
https://github.com/monossido/Hangout-isOnline with GPLv3 license
**Hangouts isOnline is not affiliated to Google**