A friendly way to say it is: 어! 너야? = oh! is it you? (only between closer friends or people younger than you) and using the person's name + 이다 is a good way too. Like: 사라예요? 사라야? 사라 씨임니다? I think these are the most common ways to say that question in Korean. ^^ I hope I'd helped you! good luck~

From what I have learned in Korean thus far you should probably say the persons name then say 이에요? Like say his name is 종현 maybe you would say 종현 이에요? Because saying "you" directly is a little offensive.