Summarize:

Find the celebrity's Twitter handle by typing in the celebrity's name into Twitter's search bar. If nothing comes up, try looking on their website or portfolio for their Twitter Follow link. If you see a blue icon with a white check mark next to a Twitter account, that means that Twitter has taken steps to verify the identity of that account. A verified account means that the celebrity in question owns that Twitter account. Be wary of unverified Twitter accounts. It's easy to find pictures of a celebrity on the Internet to use as the avatar. Fans and other people may try to impersonate the celebrity for attention or for malicious spamming. The more active they are on Twitter and the more personal their tweets are, the better your chances of getting them to engage with you. After considering the following questions, if it doesn't seem like your celebrity does much personal engagement on Twitter, you may want to consider moving on to another favorite celebrity. How frequent are their tweets? Do they frequently tweet themselves, or hire someone else to tweet on their behalf? Do they post pictures and links in addition to text tweets, or do they do the bare minimum? Do they post their personal thoughts, or just use their Twitter as a way to promote their business commercially? Have they ever sent an @message to one of their fans and engaged in a conversation with them? For example, Christina Perri not only tweets multiple times a day, but also with photos and hashtags, suggesting that she is an experienced Twitter user. Her tweets are personal enough that they probably come straight from her. Your chances of getting her to follow you are decent if you make a strong favorable impression on her, but notice how her (public) interactions with fans on Twitter are low.
Follow the celebrity on Twitter. Pay attention to how the celebrity uses their Twitter account.