Who really came over?

Regardless of who came when, your task is to identify that person (or those persons) precisely. Don't rely on family tradition. Locate documents in America that tell who the immigrant was.

Use federal census records.

For those arriving after the early 1800s, the federal census is a key source. Beginning in 1850, the census recorded where a person was born (the state or country). Wherever you can find an ancestor in an 1850 or later census, you will usually be able to tell if he or she was an immigrant.

A word of caution from previous experience.

However, be aware that the information in 1880 and later census records that report the birthplace of the person's parents may be inaccurate. Sometimes the person answering the census taker only knew the family had, say, a German background and assumed that the parents came from Germany (or another foreign country). Most persons report their own birthplace accurately, but don't depend on them to be as totally correct when talking about their parents. This is why an 1880 census might report the father as being born in Germany, and the 1900 census might report the same individual's father as being born in Pennsylvania. Between 1880 and 1900, the person learned something new about his father.