I think there are a couple of sources that could make this fairly easy to determine. He had to rent / lease Netherfield - if he did that from a local person, it makes sense to me that his income would be part of that discussion. Also, I'm sure he got servants from the local area (especially since he had no permenant country home from which to bring them), so there's a possibility they would know as well. I'm less sure about the latter one; I could see how it might come up, but it seems less sure than the first way to me!