You find the stepping printed on the IHS of the CPU. It's basically a jumble of code that, if read correctly, tells you when the CPU was made. Generally processors with similar steppings will be able to overclock about the same.
It's also mythed to include some info about what the CPU could have been made into. I.E., it could have been a 3700+ SD, but was manufactured as a venice 3000+. Therefore, it should be able to clock to SD speeds and higher.