Super Tamsin gave great answer. There is a situation where "You're hungry?" would be appropriate:
It would be appropriate to express disbelief or incredulity. For example, if I just a large meal an hour ago, and now I start nibbling on more food, you might say, "You're hungry??" In this case it has the the implication that you can't believe it.
But when just asking the simple yes or no question, "Are you hungry?" it shouldn't be shortened except as Super Tamsin said, very informally, people might ask, "You hungry?" without a verb.