Smart entrants will give you a pretty good idea of what to expect, so then you judge it according to your expectations based on their description. If they just say "dark witbier" and nothing else, it doesn't give the judges much to go on, and you're forced to judge it purely hedonistically. But if the entrant instead says "witbier with black malt, black pepper, and anise" then you have a better idea of what you should be getting from it. In an ideal universe, entrants would all strive to be that specific. But in reality, they probably won't.
So.... you'll need to search your palate deeply to try to detect all the various complexities that might be present, and describe all flavors detected to the very best of your ability, and compliment the entrant on a job well done if it works with the beer, or feel free to scold them if it makes an otherwise decent beer taste like garbage. Your gut might tell you that a "peanut butter and pineapple stout" is not the greatest idea in the world, so you are freely allowed to score it as such if the aroma and flavor confirms it for you. But also keep an open mind, because you never know when you just might be very pleasantly surprised.
The hardest ones to judge might be the "sneeble & snozberry porter" when you have never tasted sneebles or snozberries before in your life. Then of course all you can do is judge by how it tastes, just as best as you can.
One other opinion I have is: While tasting, constantly ask yourself if this is a beer that either conneisseurs or John Q. Public would positively LOVE and CRAVE if released to the public. Those beers should be exceedingly rare and deserve scores in the 40s. But if it's got no technical flaws but it's just a plain jane brew -- sure you could drink one or maybe two, but you wouldn't buy a whole 6-pack for $6.99 -- then it deserves about a 30, or maybe less. And if it just sucks, or has any glaring flaws, 20s. I like to look for if the beer is either A) elegant in its simplicity, or B) mindblowingly complex. If it's neither of those two, I wouldn't give it above about a 32.
Anyway....... you know in your heart of hearts that 99% of these beers are going to be Black IPAs anyway, and you know what to expect from that. So be ready for it..... here it comes....... that, or Cascadian Dark Ale. What a crock..... but I digress............