Maybe you could contact the neighbors under the guise that you are concerned about potential problems with the pipes and furnace and see what they say. Say something like “I’m thinking about buying in this complex but have heard that these can be issues. Do you know anything about that?”. If they pretend they don’t know about it (or genuinely don’t know) then you know it hasn’t been done. But if they have done it most likely they would bring it up if you mentioned something about it.
