There are a couple of ways to say this, depending on who you're talking to and the level of familiarity with them!
밥은? = Food? (did you eat?)
밥 먹었어? = Did you eat (food)?
점심 먹었어? = Did you eat lunch?
점심 먹었어요? = Did you eat lunch? (to unfamiliar people)
점심 드셨나요? = Did you eat lunch? (to older people/unfamiliar people) but
using '나' here is a bit more friendly and I actually hear this used
more in practice
점심 드셨어요? = The official 'Did you eat lunch?' to older
*keep in mind that I skipped the articles here (은/을) cause I never hear them used in speech