how do i know if a celebrity is online twitter?

[♥Rachel♥]

You can't really tell. There is no button that says online/offline
You can only tell if they are tweeting or following people

[Pink Freud]

You don't, unless you see them tweeting, but even that can not show for sure because some have auto tweets, or they might not be on twitter but may have their twitter account connected to an application like Tumblr or Instagtam or Vine or Youtube, where the things they do on those sites show up as tweets on twitter.
Many Celebs have other people that run their accounts, so even when you see them online it may not be them.
I;m sure you are asking because you would like to talk to Justin Bieber, you just ave to keep your fingers crossed. He may see you in the mass of tweeters.

[Excessivebait996]

I don't think they would want people to know, if they do they will offer it up themselves. Besides why does it matter. It would be much better them blending in with the rest of society , gives them some normalcy , im sure they to wish to talk about what ever it is on twitter your all talking about. Often when celebrities get noticed people fall all over them and run jump etc.. we've all seen how some people act when they see celbrities. So the same goes for twitter . Could you imagine people freaking out that so and so is on twitter chatting. Unless the celebrity themselves wish to be known I would not worry about how you can tell. Just enjoy the topics that you are all discussing.

[Meta]

Next to the tweets it usually shows how many hours or days ago they posted their tweets. So you can just tell when they are on there, like some days they will be posting loads of tweets one after the other like every hour or so. Some celebs are on Twitter almost every day, posting their latest party photos or photos of when they wake up or play golf or whatever. I think it depends on their personality, some of them just "take to" Twitter and some just aren't really interested in it.

[Joan]

when you see them tweeting, they are online

[Jae]

Blue checkmark next t their name