A casual spoken expression meaning 'How about...?', 'How did you like...?', or 'How's it going?'; used to ask for someone’s opinion, impression, or status.
この映画、観に行かない？どうだい？
Want to go see this movie? How about it?
新しいアパートはどうだい。広さは十分かな？
How's the new apartment? Is it spacious enough?
試験の結果はまあまあだったよ。どうだい、君は？
My exam results were so-so. How about you?
（電話で）久しぶり！元気にしてる？どうだい？
(On the phone) Long time no see! How have you been? How's it going?