That particular one's easy. Often a girl will say "I hate you" jokingly when she means she likes you (could be as a friend, or more - but don't assume it's more). If the body language is really obvious, then probably go with what you can understand from that. If you can't, just ask them what they mean, and if they're joking they'll tell you normally - unless they have a cruel streak, as some people do.
My friends often have to explain to me when they're being sarcastic, and they've gotten so used to doing it that often immediately after one of them makes a sarcastic comment someone will explain it to me, even if I'd actually understood the sarcasm of the comment that time!
eg.
I: "Wow, I just LOVE the vomit coloured goo they've served us tonight."
Me: "Ha-"
L: "By the way, he was being sarcastic Caitlin."
Me: "Yeah. Think I got that one."
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Into the dark...