Is your dad still a detective with 〜の

Grammar Explanation: Reasoning with 〜のだ

Ran seems angry as Shin’ichi basks in the media attention he’s getting for solving a difficult case. When Shin’ichi asks Ran why she’s angry, she points out that it’s because of Shin’ichi that her father’s been getting fewer case requests.

蘭(らん):: 「怒(おこ)ってなんかいませんよー！！」; "I'm not at all angry!"
新一(しんいち):: 「あれー？蘭(らん)の父(とう)さん、まだ探偵(たんてい)やってたのか？」; "Huh? Your dad was still working as a detective?"; Literal: "Huh? Is it that your dad was still working as a detective?"

Shin’ichi has the incomplete observation that Ran’s father is losing work taking cases because of Shin’ichi’s detective work.

Considering how bad Ran’s father is at being a detective, it’s surprising to Shin’ichi that he’s still in the detective business.

However, based on Ran’s remark, Shin’ichi reasons that her father must still be working as a detective. He puts this as a question for confirmation.