Possibly the only evidential proof of a missile strike will be shrapnel recovered from the victims at autopsy.
Isn't there any chance that ATC Radar will have captured the missile's path from launch to impact with its target. Of course as the Ukrainians have not so far come up with anything then perhaps not. No doubt ATC monitoring facilities in that region are still primitive in the extreme.
However this is unlikely to be tried in any court of law for a long time but the unofficial trial of the rebel separatists will come in terms of the sanctions and other pressure imposed on Russia by the West. Since the separatists and the Russians (who quite unbelievably gave them these weapons without providing the supervisory personnel to ensure that a mistake like this dd not take place) are blatantly guilty unless they can prove otherwise the same rules of evidence as a court needs to send someone to prison are not going to be required to impose sanctions on Russia or cancel the 2018 World Cup in order to punish Putin personally.