For Intel CPU's, if you look at the Processor Spec Finder
and put in what is on the box, like for example Q6600:
You can find the different steppings, as well as the sSpec#, which you can tell that a SL9UM is a B3 stepping, and SLACR is the G0 stepping.
The sSpec# is on the box it came in (retail) IHS as well, which would even be on an OEM CPU.
Not sure about how AMD is, since it's been a long time that I've own one.