It is considered impolite to ask someone else if the "want" to do something. Instead, use the negative form of the verb in a question.
Very important: "tai" can only be used for the I person, like "i want". If you want to say that someone else wants, you use "tagaru". Ex:かれは牛乳を飲みたがっています。 This is because Japanese never imply they know what someone else wishes, and "garu" means: "i think that", "i heard that", etc.