I don't think most debates are "won" or "lost" in a conventional sense. If both parties walk away understanding the other's viewpoint better, then both have won. If both parties just argue without listening to the other side, then both have lost. That is how most debates are either won or lost.

In a more formal setting the winner is the one who persuaded the most neutral people to their viewpoint. Like in a political debate the winner is basically decided by people who were undecided voters before the debate.