It can be proven with authentic autopsy reports & pics of the body however you think such stuff won't cause a massive riot over here ? just imagine what that gangrape case did to Delhi now multiply it tenfolds & spread it across the nation ~ it'll kill Ind/Pak relations for the next decade or so, if people have their way !you indians want to believe the beheading took place rather than want to verify that it took place, this is the single biggest issue.
its a far fetched story, just verify it, no sane pakistani would be proud of this, including the army, so just prove it, its too big a deal to NOT offer proof, therefore you need to prove it.