This is a very easy test for finding out whether your friend is a zombie or a dick. As your friend approaches you, moaning and groaning, squat down below the reach of his outstretched arms and quickly punch him in the groin. If he is truly a zombie, your punch will likely elicit a non-reaction as zombies don't feel pain. Then it's time to run. But if your friend is only faking being a zombie, he will crumple over in intense pain, righteous justification for acting like a dick.
My use of masculine pronouns is not a generalization. Girls generally don't act like dicks, and almost never would act like a zombie dick under most circumstances. If a female friend is acting like a zombie, it would be best to incapacitate her as quickly as possible. Decapitation is usually a viable method.
Ed. note: This method also guarantees that no one will try being a zombie ass again, so if your friend is lurching towards you a second time it's best to find the nearest heavy, blunt object and whack them over the head.