"I find out who is my father."
Translation:Tôi tìm ra ai là bố của tôi.
I find out who my father is. <= correct grammar, although present tense is super unlikely for this sentence.
Present tense is not "super unlikely" if the presumption is that the father is still living. If he is dead and gone, "was" is more likely. "Was" might also be used if the sense of the sentence is "Who fathered/begot me." The tense is dependent on the point of view of the speaker. "Was" never entered my mind until I read the comment and tried to think when, if ever, I would not use present tense in this sentence. (Native speaker, American English, which does not observe sequence of tenses as rigidly as British English. )