How do I say "You are lying!" in a joking way?
Can I say:
or is there a friendlier way to say it?
嘘だよ is likely to mean "I am joking." One way to say "you are lying" is to use an interrogative form:
嘘だろ！？ / 冗談だろ！？
Isn't that a joke?
Examples above are very casual. Of course we can make them formal by using 敬語:
嘘ですよね？ / 冗談ですよね？
(note that マジ is a casual saying of 本当).
If we use a normal sentence, it might sound in a strong tone, so not friendly:
嘘だ。 / 嘘だろ。
It's a lie. (it may sound like "It's definitely a lie.")
Perhaps a little more indirect:
Often translated as “it’s different”, it’s also used for “just kidding” or “that’s incorrect”.
In Japanese culture, people rarely confront each other or directly disagree. Most of the time, they will correct a misunderstanding rather than disagree.