Here are some example how you can say what you want. These are the ones that I know directly, so there might be even more. And for each how rude is it:
Du willst mich wohl verarschen.
Really rude. Unless you love pain, don't say that to somebody. Or at least to somebody that knows how you mean it.
Willst du mich für dumm verkaufen?
It's better, but it shows a really good amount of being upset. So unless you aren't upset this is not what you're looking for.
Da nimmst du mich aber auf den Arm.
Nimmst du mich etwa auf den Arm?
Du nimmst mich doch auf den Arm.
All of them show that you think your opposite didn't tell the truth. Unless you don't pronounce it as being upset it's a common neutral and thus bit humorous way to tell people you got it.
Das glaube ich nicht. (Guter Versuch ;-) )
Simple and direct way to tell somebody you don't believe him. "Guter Versuch" it's kind of a funny way to show that you understood it was a joke and are not offended.
So there are different ways. All of them can be used with "Sie", too. But the direction don't changes when replacing Du by Sie. I would in most cases tend to the last one. Simply show in simple words that you got it, it was a joke and you aren't offended.
Last but not least here a really uncommon way I remember and personally use lots of times:
Mein Ironiedetektor kam zu keinem eindeutigen Ergebnis.
In my opinion it's a really funny way (and even not used by anybody else) to tell that you aren't sure if it was ironic or not. (Note that
Ironiedetektor is not a real word. Source: unknown)