where is the proof?
You have yourself stated the answer. It is very difficult to get the evidence against a nation. It is much easier easy to get it against an organization. As you have to be 100% sure otherwise will lose face in International diplomacy as a ranter. And even in cases there is evidence like ISI attack on our Kabul mission, the US pressure just made us sit and do nothing. Even though a stern warning was given by US to PAKMIL/ISI but nothing happened even after much fanfare when even Gilani said that we will be replacing ISI chief with a civilian. This is because Pak has got a strategic edge now a days and unless Afghanistan is solved, US would not let India take overt actions. Thus covert actions are our best bet and even the US is doing it's covert actions in Pak to make PAKMIL move to N. waziristan. It is part of the game and nobody can provce it.