Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
I want to know if a user is online at a given time or not. So that commenting on a question or answer would be effective.
Because the user profile only shows that the user was last seen : (Ex:30 minutes ago) but he/she may be online now.
Can you know? No.
But leave them a comment (using their @name if applicable) and they'll get a notification when they are online.