You judge a beer based on visual, aroma, taste, mouthfeel; and descriptive terms generally fall into these categories.
Visually you might describe the color of the brew (amber, golden, inky black, hazy, clear), the color and retention of the head, how the beer poured. Aroma might be described by smell of hops (floral, citrusy, resiny, piney, etc.), or malts (caramel, toffee, smoke, chocolate, etc), or other aromas present (molasses, booze, soy sauce, oak).
Describing taste opens up even more potential terminology, but don't be shy to use whatever terms or comparisons best describe the flavor for you. Think of tasting beer as an evolution of flavor in your mouth: describe how it hits your tongue; how the flavor builds, morphs, and leaves; and the aftertaste. You might use any word you would to describe the flavors of food and more to describe beer (Sweet, salty, sour, tangy, raisins, dough, bread, bacon, spicy, herbal, etc).
Finally, for mouthfeel, you're looking at words describing the texture and consistency; so ones like smooth, rich, chalky, gritty, effervescent, thick, syrupy, etc., are very useful when appropriate.