Say your run of the mill HMS Frigate rolls up on an American merchant vessel and her boarding party demands to "reclaim" all British-born able-bodied seamen. How would the birthplace/citizenship of individual members of the American crew be identified?

From reading Ronald Utt's Ships of Oak, Guns of Iron , it seems to me that the British, who insisted that their impressions were limited to those they believed owed loyalty to the Crown, must have claimed a use of some sort of system to distinguish the sailors in question from other sailors.