Welcome to Android Central! Are you using Android Device Manager? With that feature, you can't see who's using it. But if it successfully located it, you could try remotely wiping it.
For future reference, some 3rd party security apps have features that might help identify a thief. For example, Lookout can take a photo using the front facing camera if someone fails the unlock PIN/pattern 3 times in a row, then emails the photo to you.