Generally, rats use their amazing smelling ability in order to locate sources of food.
Rats are also very smart for their size, once they have found food, they will memorize its location and return over and over again for another meal.
Rats can also find food by using clues in their environment, such as the presence of rat droppings or the smell of other rats which may indicate the presence of a food or water source.
Rats can also bring food items back to their nest, storing the food underground for use at a future time.