Estne Marcus puer probus?
-ne is simply an ending that indicates the sentence is asking a question. So if you ignore -ne for a minute, what does the rest say?
Marcus is a good (honest) boy.
Now turn that into a question and you'll have the real translation.
Estne means is (To a point), Marcus is a name, and puer probus (Next to each other) means good boy. Put it together and. . .
Ad quod quaeris, nescio.