Besides the end points, we also know (without trial or error) that virtually all points between the ends are more than zero, and that they're not all equal, which means that there has to be a maximum point in there somewhere. The way we know whether we're on the right side of the maximum (excessive rates and diminishing returns) is by cutting taxes (we did) and seeing if revenue goes up (it did).
Tell me if I'm missing something, but it sure looks to me that this proves that before the cuts we were over taxed.
And why hasn't Bush been shouting that fact from the rooftops at every opportunity?