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Most Helpful Guy
There's no way but you can have a rough idea if their profile is open.
Open their profile and check their last seen.
It gives you an idea that the were/are online in last hour.
To increase your accuracy, check their recent activity too.
As shown in above pic, the person's last activity was just one minute ago which means he is online.
If their profile is private. Best you can do is follow them and repeatedly refresh the site feed. If you can see their activity, he/she is online.
Good luck :)1