When did Luke realize Vader was telling the truth?

When Vader told Luke that he was his father in ESB, Luke initially didn't believe him. Then when he was aboard the Falcon, Vader called out to him, and he answered "Father". So when did he realize that what Vader told him was true?

 

I think that Luke always knew. Vader did tell Luke to search his feelings so maybe from that advice Luke could deceipher the answer. And after Yoda told him the truth, his doubt was finally laid to rest.

 

Vader: YOu know it to be true!

Luke: Crap! You're right! (Outer dialogue: Nooo! Nooo!)

 

Luke knew the moment Vader told him.

 

He knew right away, he was just in stunned disbelief. Once he had a few moments to gather his thoughts on the Falcon, he knew it was true.

 

I agree. I think he could just sense it.

But then why did he ask Yoda? I guess because the viewers wern't shure.

 

Like most of us, even when we know something is true we still
need confirmation most of the time.

 

He knew as soon as he "searched his feelings".

 

He realised when he was hanging outside of cloud city.