As you have told us that this appears on the beginning page of a book this is a little quote to indicate the topic of the book.
A philologist is someone who studies language, particularly the classical languages of Greek and Latin.
Nietzsche asks (Enquires) about how philologists start their study (the Genesis of philologists)
His reasoning is "Young men can't know about the Romans" therefore "A young man cannot know if he will be good at finding out about the Romans" (and therefore no young man will choose to become a philologist)
He suggests that this is some kind of "chicken and egg" situation. But this doesn't seem to be anything very deep, more a joke by Nietzsche:
Why would anyone study Latin unless you know you are going to be good at it. And how do you know you will be good at it before you study it?