Attach lots of sensors to lots of axons, try to emulate the thing while it's running... for me, it's the on-line method that sounds more plausible compared to the "look at axons with microscopes and try to guess what they do" approach. Nevertheless, imagining a scenario with non-destructive uploads... how many times would you allow people to upload? Ending up with questions like that, I think it's the destructive one that would generate less horrifyingness...