Before I have the game in my hands:
Escapist previews, Reviews on the Run, X-play, Gamestop summary info, game website, company history. The thing I pay the most attention to is gameplay videos, I'm at the point where I can evaluate the game mechanics as well as separate it from the skill of the player.
At that point if after I have all the information I want and I am still interested I hope for a demo to get some hands on time.
If I still like it after the demo, or there was no demo chances are I'll rate it in my buying system(by factoring in all given information), which essentially filters it as a pre-order must have, early release buy or a price drop buy.
Once a game has made it through all of that I generally have something I do really like or I am at least happy with the amount of money spent on it.
I have really only been let down once and that was for Alpha Protocol, through all the hype I was thinking Mass Effect+James Bond but the AI really crapped out on me so I returned it for full price after having purchased it at the pre-order price
My list of current "safe" must pre-order games now consists of Portal 2, Mass Effect 3, Gears of War 3, this is due to previous company history, previous series history and media releases by the companies.
I am taking a gamble (gamble to me meaning there have been no previous titles in the series by the same company and only going by other info gathered) by pre-ordering Hunted: The Demon's Forge just because all of the info I have gotten so far looks good including the hands on gameplay videos.
Successful new IP pre-order "gambles" included Assassin's Creed, Borderlands and a few others.
TLDR: I judge a game through a long series of checks from news hype, reviews, game play videos, demos. And once I get it in my hand I generally have a good to awesome game.