A German surname ending with -li almost always comes from
Switzerland, or at least southern Germany.
The ending -ski is often from Polish areas.
Patronymic endings such as -son or -sen suggest Scandinavian
areas, or if German, the province of Schleswig-Holstein.
German families with French given names may be from
areas such as Alsace-Lorrain, on the French side of the Rhein River.
Prefixes such as Mc- and O’ in Scottish and Irish
surnames suggest a specific clan, which usually had an ancestral area
within their respective countries, although the immigrant may not have
been raised in that area.
This approach can be quite complex, and usually requires
the assistance of persons well versed in the language and culture of your
immigrant family. Sometimes talking the subject over with a local university
or college language department will help.