I think trusting a game reviewer to be honest is rarely a problem. The fact that review scores vary so greatly supports the idea that people are being honest. Once everyone across the internet and beyond start giving all games the exact same score, then it might be time to be suspicious.
However, I think the question being asked is closer to, "How often do you trust that a review you read is going to reflect your own opinions on a game once you purchase it (if you do)?" In this case, I often stick to reading reviews from people with game tastes that are similar to my own and base most of my purchases on what they have to say. For some reason, I seem to like the same types of games as Ben "Yahtzee" Croshaw, from Zero Punctuation fame, so even though his reviews are mostly humourous rhetoric, I still find them surprisingly useful. I also find my tastes falling in line with Adam Sessler's from G4TV most times. Plus, I find Metacritic occasionally helpful. I know a lot of people chastise it for trying to make something quantitative out of a giant conglomeration of opinions, but it is a nice way to get an idea of what the general attitudes on the quality of a game are. The greater the number of game critics that like a game, the greater the chances that I might like it myself.
Given that everyone, even people from the same site, are going to review certain games quite differently than others, I think the only way to get any kind of decent consistency is to stick to a group of reviewers that share some of the same tastes in games as you do. Really delve deep into what they say in their reviews, as well as what they write elsewhere, and see if what they are saying sounds like something that might come out of your own mouth (or fingers). If you love football and a reviewer for a Madden game clearly states that he or she does not, that review is going to be useless to you, and you will have used your time more effectively by searching for another review than giving that one a second thought.