I got a 2130 on the SAT (750 math, 690 reading, 690 writing) my first time taking it, and didn't do any preparation/practice for it.
I got good grades (A's and B's) in high school (mainly in honors and AP classes) with little to no effort - the only classes I studied for were biology and AP US history (since they required memorizing a lot of details).
In college, I majored in computer science, got minors in math, philosophy, and cognitive science, and graduated with a 3.2 GPA - I put more effort in than in high school, but still didn't try very hard.
I work as a programmer analyst now, and was told in my performance review this summer (about a year and a half after I was hired) that my results are second to none.
I'll let you draw your own conclusions from that as to whether or not I'm smart.