That's a very good question. I can't speak for every film critic, but I can speak for myself as I often review movies on here. During my early years on hubpages, I'll admit that some of my reviews were a bit biased. However, as time went on, I gradually started to get better and better at reviewing movies to where I'd like to think that's no longer an issue.
The things that helped me was that I did study a lot of writing styles of various film critics, and often tried to mimic their strengths; while learning from their mistakes. Another thing that helped me was that I remember opening up a few forums on here once asking why people don't like film critics, and I will admit most of the criticism towards them are rather harsh.
Most people generally assume that all critics have the same opinion, while others claim they're pretentious cinema snobs that devalue good movies, while praising ones that people rarely see. That's why I try to present myself as more of a movie fan these past couple of years more so than an actual film critic, to eliminate that perception.
I also take in advice that my father once gave me a while back, that it seems most film critics don't follow, which is judge a movie for what it is, and not for what you expect it to be. If you honestly analyze as many reviews as I have, you'll learn that most critics fall into that same trap a lot. Take "Man of Steel" fro instance. Many film critics hated it because it wasn't anything like the classic George Reeves or Christopher Reeves' superman back in the day. They wouldn't accept that man of steel was a new interpretation of the character from the modern era of superman comics. However, since the movie was nothing like what they wanted to see on the big screen, they automatically denounce it as crap. Not saying that every film critic did this, but a lot of them did. I won't mention any names, but if you look through all of them, then you'll see what im talking about.
This is why I always try to look at the filmmakers intent behind every film. Was the movie supposed to not be taken seriously? What was the filmmaker trying to say? What does this mean symbolically if there is a meaning at all and etc? Stuff like that. From there, I analyze whether or not I feel the filmmaker accomplished that task.
After that, I even factor in target audience perspective on whether or not they might like the film; which is something no other film critic does. Anyways, that's what helps me.