It isn't necessarily possible to tell whether somebody is Jewish just by looking at their last name. Although many Jews have last names that sound typically Jewish, many Jews in the United States have English-sounding or German-sounding names.
Although many Jews have surnames that are distinctly Jewish-sounding, such as Klein, Weiss or Stein for example, these are in fact German names and do not necessarily indicate that the person is Jewish. According to jewfaq.org, the third most popular last name among Jews in the United States is 'Miller' – a name that is also common among Gentiles. Some names are almost always used by Jews, such as Cohen, Katz and Kaplan, as ehow.com points out. It should be remembered though, that historically Jews did not bear surnames at all, until relatively recently as jewfaq.org intimates. Therefore, it is understandable that Jewish last names would reflect the eastern European origins of the communities among whom the Jews lived.