I have a question about empiricism. If I was an empiricist how would I know that
I think the simplest answer is this: empiricists think beliefs about matters of fact should be grounded in empirical evidence; they don't think the evidence always has to be direct. I've never been to New Zealand, but I have a considerable amount of indirect empirical evidence that it exists. To take just one bit of that evidence: people whom I know to be otherwise reliable and honest tell me they've been there.
On the other hand, I don't even have indirect evidence that Middle Earth exists. What I have is evidence for is that J. R. R. Tolkien wrote stories about a fictional place called Middle Earth, which neither he nor anyone else claimed was real.
The (small) larger point is this: the one-word name of a view is not always the best way to figure out what the view actually comes to. Very few, if any, self-described empiricists have said that evidence always has to be direct to be relevant.