If ‘something does happen’ to Julian A., a practiced criminal would do his best to conceal that event for as long as possible. The event should remain unknown to any online contacts Julian A. has been in constant contact with. A practiced criminal will have tech assistants ready to continue the online Tweets, Facebook entries, and other, barely skipping a routinely expected beat of contact.
Someone would have to be able to observe Assange over time to see what he usually does and with whom. Is he usually sanguine and calm in emails, or does he tend to rage and about what?
The Soviet way would have involved slipping some silver or polonium as in the case of Alexander Litvinenko. But that process takes a long time to work it’s damages.
As far as I know, Assange is in the Ecuadorean Embassy in London and does not leave the building. If he dies, people would know pretty darn quick.