I've never really understood it when people talk about CPU steppings, and how one is more OC-able than another stepping. And how do you know what stepping you have? Or if you are going to buy a CPU how do you know what stepping you're going to get?
An AMD rep once explained to me that it is like a revision level that represents when it was manufactured. I heard years ago that dual / quad processor systems needed to have processors with matched stepping to work correctly. I hope someone can provide additional detail.
It has been the case in the past that some steppings have overclocked better than others, tho this is only an average and has to be taken on a chip by chip basis, every stepping will have good and bad chips.
As for knowing what you are buying, well ask at the time of purchase. It is only an issue normally when a new stepping has just been released.
To check you current stepping, there are several programs. The one I use is CPU-Z.
It tells me that my Core2 Duo is a Stepping 6 rev B2 chip.