Informal:
* 뭐 봐? (mwo bwa?) - This is the most common and casual way to say it. It literally translates to "what are you seeing?".
* 뭐 쳐다봐? (mwo chyeodabwa?) - This is slightly more aggressive and implies that the person is looking at you in a rude way.
Formal:
* 무엇을 보시는 거예요? (mueot-eul bosi-neun geoyeyo?) - This is a polite and formal way to ask. It literally translates to "what are you looking at?".
* 무엇을 보고 계신가요? (mueot-eul bogo geshin-gayo?) - This is also a polite and formal way to ask. It literally translates to "what are you looking at?".
Note: You should choose the appropriate level of formality based on your relationship with the person you are speaking to.