It's a very colloquial way to ask "really, is that true?" (with a slight sense of disbelief, sort of like "no way!" or "you're kidding!" in English). For example, if someone claimed to have scored a perfect 100 on a very hard test (that almost no one gets perfectly), you might say, somewhat doubtingly, "真的假的?”
Your intuition is right about what a longer form of the expression might be. It could also be a shortened form of "是真的還是假的???”