If there's no article in front of a Hungarian noun, it's indefinite. You could add the indefinite article too: "Ki lát egy almát?" and it will be roughly the same. If it was "the apple", the definite article would be necessary in the Hungarian sentence as well (along with the definite conjugation): "Ki látja az almát?"
I think as well this is a bit tricky. It's not a sentence you'd use. You'd rather say: Ki lát egy almát? But good to know the rule, thanks for writing it Shamarth