Have to say that I'm with the other chaps on at least one of these points.
Making out is not and never has been synonymous with "having sex" and if you have heard the term used in reference to sex, then the people using the term were quite simply not using the right terminology. They might well mean the people had sex, but they shouldn't have used "Making out" to describe it.
"Hooking up" is a much more vague and nebulous term, and can mean anything from "just meeting up with someone in a social setting" ("Want to hook up after the pictures and go get something to eat") to actually having sex ("I hooked up with this guy last night"), but that very ambiguity means that it can't be used as conclusive proof of anything without further clarification.