how do i know if a celebrity is online twitter?

[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.