A lot of us do it, maybe not always consciously, but we tend to judge others before gathering all facts. We base our opinions based on someone's looks, we base our opinions off of hearsay, we base our opinions based off of our own past negative experiences.
A classic example of this is the George Zimmerman/Trayvon Martin case. I've read comment after comment, not to mention all the media coverage, that has judged, tried and convicted this guy without concrete facts and evidence. Most commenters take what media says as gospel truth and continue to base their opinions according to that. It's always best to delay judgement until all facts and evidence are made clear.
It's also easy to get caught up in judging someone by their appearance. We see someone that may not align with what we consider okay and immediately judge them as unacceptable. Some women see another very attractive woman and immediately think "she thinks she's something." We don't stop to think that we may be wrong about our assumptions.
A lot of times when we prematurely judge someone, it's out of some kind of internal threat. Sometimes it can be because that person triggers something about us that we are reluctant to address. Maybe the next time you find yourself judging prematurely you could try exploring what's at the core of those thoughts.