Entity ID?

I've got a silly question. How can I test what type of entity a given entity is? For example, if I wanted to do something *only* when a zombie spawns, how can I determine that the entity spawned is, in fact, a zombie and not a pig, or a cow, or so on?

I've included a picture of my dilemma below. Basically, I'm asking what I should put in the "if" portion.

Thanks :)