"Where you at?"
=> That one can be account for by phonetics. The missing verb are, when contracted (are => 're) is hard to pronounce after "Where" (Where're), so it's often either half pronounced and barely audible, or it's omitted altogether. We kind of know it's there though - I think.
The problem some people might have with "Where you at?" isn't the missing verb, but rather the fact that it ends with a preposition, "at".
"If I would've known..."
=> You know, I'd say, "If I had have known.
=> Isn't it synonymous with elementary school? Well, maybe not in the USA.
All the best.