Greet the person respectfully. Everyone loves the phrase "what's up", but honestly, it's gotta go. Because those of you who use it, what's the answer you always get? "Nothing much, you?" or "nm u?". So think outside of the box, and try something more along the lines of "What are you up to?" "Whatcha doing?" Something playful that will catch their attention. They may answer something like "Hanging out", which is perfectly okay, because at least you know that they're hanging out and not dying in the void of "not much"-ness.