知ってたの

did you know?

→ 知る
phrase Rare #21,857
did you know? A casual form of 知っていたの. The subject is understood from context; の gives the question a conversational or explanatory tone.
JA: 彼が来ないって知ってたの?
EN: Did you know he wasn’t coming?

どうして僕の親を知ってたの?

How did you know my parents?

トムに息子がいることは知ってたの?

Did you know Tom had a son?

トムは答えを知ってたの?

Did Tom know the answer?

トムがオーストラリアにいたこと、知ってたの?

Did you know Tom had been to Australia?

前から知ってたの?

Did you know all along?

あとで知ってたの?

Did you find out later?

— ア・フュー・グッドメン (1992)

あの人のこと知ってたの?

Did you know about that person?

財宝のことを知ってたのよ

They knew about the treasure.

— パディントン2 (2017)

知ってたの?

Did you know?

— ジョジョ・ラビット (2019)

彼、あなたが彼のこと大好きだって知ってたのよ。

He knew that you really loved him.