Yeah, the stepping is the version.
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.