^^ That won't necessarily prove that they have a genuine Twitter account, though. As with Facebook, Twitter is riddled with fake and pseudo accounts. Most of these are set up by "friends" of the alleged account holder who are just "having a laugh", though sometimes with more malicious intent...
You might also wonder why someone would want to know this anyway! Perhaps the OP is doing a survey, or simply wants to follow the person in question. Either way, the "Do you have a Twitter account?" seems the only way...