Steppings can only be found on the Internal Heat-Spreader on top of the CPU. It is usually a 5-letter code. For example, CABNE.
Also, there's no way to tell how far a CPU will overclock without actually doing it. Some "good" steppings overclock poorly, and some "bad" steppings overclock well. One can establish the desirability of a stepping by looking at all overclocks of that stepping, compared to others.
The thing is, if you've already got the CPU, it doesn't matter what stepping you've got. See how far it will OC before selling it off because "It's a bad stepping". BUT if you were allowed to choose from any stepping you wanted, THEN it would matter.
Usually, however, there is no choice.