How do you know when to use "is" or "are"?

When deciding which of the verbs "is" and "are" to use look at whether the subject is plural or singular.  If the subject is singular use "is," if it is plural or there is more than one subject (compound subject) use "are."

Examples: 

The cat is eating all of his food.

The cats are eating all of their food.

The cat and dog are eating all of their food.