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.")