For newer games: Drive Thru RPG or Amazon for quick reviews
For older games: RPG.net
For more in-depth reviews: I search for them directly and usually end up on someone's blog.
There is an element of risk here: For a $2 PDF I don't need a lot of depth, but for a $60 hardback I want a pretty good breakdown of what's inside.
A few more tips:
- I find later reviews tend to be better than the first wave. That initial wave of reviews seems to be colored with not-always-justifiable enthusiasm because it's NEW! You don't have much choice when looking at a brand new game but if it's been out a while and is just new to you, well, you will likely have at least a few to choose from.
- I find people who played earlier versions of a new game will give you a lot more information on whether specific problems have been fixed than people new to the game or setting. You can get good information form both but it's often very different information.
- I weigh reviews based on actual play far more heavily than "I read the book" reviews. I've read a game that looked great on paper and played horribly, and I've read books where the game looked like a mess but once we started playing it just snapped into place. Don't just tell me what the table of contents lists - tell me how it plays! At the very least how about you try to make a character and describe how that went!
It's not enough to just read the book - you have to play at least one session, preferably more like 3+ sessions, to really get a feel for how a game runs. Those are the golden reviews.