Two observations,
- Assange would have to be physically removed - if either captured or killed. If any actor has him, he is more valuable alive - ergo he is alive.
- The Ecuadorian embassy has first hand knowledge of his status and would have to be directly complicit in any bad scenario so no 'raid' would be necessary.
It seems like no proof of life is possible if Assange is determined to warrant sufficient resources. Consider this:
Here is a fake video -
done with something superior to this software -
*Osama Bin Laden style look-alikes are clearly feasible and used by intelligence agencies
*DNA testing requires having a prior sample that has maintained a trusted chain of custody until it can be compared to a new sample and that sample has to be done by some place that can somehow be proven trustable.
Perhaps the most usable proof is the continued release of leaks that harm the prime culprit candidates.