The Wikipedia article, Former Eastern Territories of Germany, has a good discussion of territories that were transfer to Poland and other countries.
As to finding out where your family came from without some document, family letters, or perhaps family tradition to point to an area in modern Poland or Russia, or finding them in the extant records would take an extraordinary amount of luck.
There are some sources that can give the Polish or Russian equivalent of former German place names.
Knowing your ancestor’s religion is important. Poland is largely Catholic, but Evangelical is common in Northern areas. There were also many Jewish areas. Poland suffered greatly during World War II, so much may have been lost.
I assume you have not found the family in the passenger lists. If you have not yet looked, try Steve Morse’s “one step” web pages at https://stevemorse.org/