We can watch people play the game, streamed live over the internet, and see it for ourselves.
Or, if your streaming is poor, your timing is off or you just like keeping this window open, you can watch replays of some of this week's DOTA 2 debut games, captured from the official DOTA 2 tournament stream by the people hosting the thing.
Enjoy your look at the game in action.
(I picked two matches at random from the five replays currently listed on the official DOTA 2 YouTube page. You can go there and queue up any others you'd like to see.)