The best clinical instrument for 'measuring' sociopathy is the Hare PCL-R, which tries to estimate both a person's tendency to engage in hostile interpersonal behavior and their attachment to social norms - you could think of it as the difference between actively murdering people vs. callously watching them die without helping. One can have a high score on one part of the scale and not on the other. Obviously this is far short of being an exact science.

As for empathy, it might be an inherent quality but I'm inclined to think that it's like most other things, you can cultivate it with practice and it manifests different ways in different people.