Related question on Sci Fi SE: Did Calvin ever realise that Hobbes was not real?
I do realize that this is a somewhat old question, but I'm first going to challenge the premise of the question a little bit: what do you mean by "real"? Real in what sense?
If by "real" you mean "what would a video camera show?", that's ambiguous by design (as some of the other answers have pointed out). Watterson deliberately didn't resolve that in the strip; he consciously tried to make it not be provable either way from the strip. In fact, even in a strip where Calvin takes photographs of Hobbes, Calvin and his dad just see different things, so it doesn't really resolve which view was "true."
I think that the important thing is the characters' point of view, not who's "correct." In fact, I'm not even convinced that the question is even meaningful in terms of the strip. From the perspective of Calvin's parents, Hobbes is a toy; from Calvin's point of view, he's real. You could argue that there's a real sense in which the different points of view are equally "correct" in terms of the universe of the comic strip. Either way, it's not important, "provable," or even necessarily meaningful to ask about which is correct.