You call someone a "smart ass" in a very specific situation:

1. You say something that's not supposed to be taken seriously. For example, you exaggerate something or speak sarcastically.
2. The other person knows that you're not being serious.
3. However, they act like you were being serious anyway.

In this situation, you can say that they're "being a smart ass". Here are some examples:

A: I love this restaurant!

B: You love it? Why don't you marry it then?

A: Don't be a smart ass!

A: I'm so hungry I could eat a horse!

B: Oh really? Do you want me to butcher one for you?

A: Whatever, smart ass!

You can also call someone a "smart Aleck". The meaning is the same, but it's a little more polite.