Generally, family tradition, public records or the nature of the family name will reveal an ancestor's country of origin. Then a researcher needs to identify the exact parish, town or county from which he or she came, in order to continue research there. After gathering this information, a genealogist can go to the library and make a survey of the surname in that country. If an ancestor was French, for example, research begins by looking for works dealing with the surname and seeing whether any of them mention the part of France in which the name originated. If books dealing with the surname can't be found, general works of French surnames may be helpful. Check the library catalogue under "France Genealogy" for additional sources or do an Internet source using the same keywords.
The genealogist should also study histories of the country from which an ancestor emigrated and, being realistic about his or her station in life, and trying to imagine the ancestor within the context of that history. What influence did that history have on the ancestor's life? Without historical knowledge, a genealogist can't perceive the causes and effects that influenced an ancestor's life and affected the genealogical data being sought. Every shred of information on this should be uncovered.
While pursuing library research, a researcher might write to the national archives in the ancestor's homeland and ask whether they have any knowledge of the regional origin of the family name and whether any indexes of surnames exist. England has Boyd's Marriage Index and other recent marriage indexes based on entries in parish registers. These indexes are most likely to lead to the area from which a family came if the surname is somewhat unusual. But in the case of common names, they're of little value. At the same time, a researcher can ask whether that government has published a guide to genealogical research and how to obtain a copy.
In instances where it's difficult to determine even the national origin of an ancestor, Elsdon Smith's The New Dictionary of American Family Names and other general works on the history of surnames may offer clues. Each genealogical case is unique, and every lead should be pursued in hopes that one will reveal the name of an ancestor's foreign home.
While genealogists are anxious to get their feet wet by crossing the water, they must complete their research in this country before traveling to others. They must be absolutely sure they've examined all of the data necessary to pinpoint their ancestors' homes abroad.