Not all Carnivores are Obligate Carnivores

However, not all members of the order Carnivora are obligate (obligatory) carnivores.The difference? Take bears, for example. While bears kill and eat flesh, most of their species are omnivorous (eating both animal flesh and plant material.) We humans are omnivorous; so are dogs. Omnivores can live quite well on a combination of both meat and plant foods. Our intestinal tracts are quite long, and can do a satisfactory job of digesting, extracting the nutrients we need, including protein from legumes, rice, and some vegetables, and eliminating the rest as waste.

Why are Cats Obligate Carnivores?

Cats "guts" are much shorter than ours. They do not have the ability to fully digest and utilize the nutrients in plant material. Although theoretically, they might get enough protein from plant material to exist, they need taurine in order to thrive. Taurine is found primarily in the muscle meat of animals, and is most highly concentrated in the heart and liver. In the wild, cats may get a small amount of grain and other plant material from the stomachs of their prey, but our domestic cats really do not need large amounts of grain. Corn is a good example. Corn is a cheap source of protein, and many of the "supermarket brands" of dry cat food are packed with corn in various forms, e.g., corn bran, corn germ meal, ground corn, corn gluten, corn gluten meal. If I could give you one rule of thumb only, it would be to shy away from any cat food containing corn, especially listed at or toward the the top of the ingredients. (Pet food ingredients are generally listed in descending order by dry weight.)
It should also be noted that some manufacturers practice "splitting" to keep corn from the very top of the list. Splitting is carefully calculating the percentage of each kind of corn so that the aggregate total weighs more than any other ingredient, however none of the corn ingredients is listed at the very top. There is nothing illegal about this practice, but it puts the consumer at a disadvantage. It would be much more transparent if manufacturers were required to list the percentage by weight of each ingredient. Corn is not only a poor source of protein, it is a known allergen to some cats.
While some cat food manufacturers add small amounts of vegetables and fruits to their formulas, the premium foods always have named meat protein high on the list of ingredients. The fruits and vegetables are generally added for their vitamin and mineral content, and sometimes for their probiotic value.