How do they determine who wins a presidential debate? ?

It is done by pollsters who call people at random. I'm sure you can vote in a cnn poll online or something like that. But for a poll to be accurate it has to be done at random. Gallup.com is the best Polling company.

Edit:

The most important thing in a debate is the public opinion of Americans the next day. So the news reports the winner of the debate based on what the polls say. There is not a score sheet or anything like that.

It used to be based on the rules of debate and the content and strength of the arguments.

Today it is all opinion. Unfortunately, nobody in the media truly knows how a debate is scored and they base their results on the opinions of the pundits who by nature are partisan based.

So, to answer your question, the winner of modern debates is at best a guess based on who made you feel better.

The actors in Hollywood phone in and tell them.

It's all really a matter of opinion...who you think did better.

Naturally, Obama supporters will think he did better, McCain supporters would argue for McCain. it really doesn't matter besides what you think.