Some of the difficulty comes from the fact that many people keep their surnames from other countries.
One thing I can say about true English surnames is that they often have roots in a place or profession. Hundreds of years ago, people's surnames were given from what they did. For example, "Farmer" and "Smith" are two common surnames that began this way. So if you see words you recognize in an English last name, you are probably safe to pronounce it like the word you know.
Also, it is not considered rude to ask someone how to pronounce their last name if you have never seen it before. They will probably be able to tell you are learning English, and many people are actually quite happy to talk about their family name.