A slightly different topic, not quite ethnicity, is that of cultural background. In the present life the culture I belong to shares and overlaps with a past life culture, but there are significant differences between the culture into which I was born and my past-life culture.
One of the things which I've found useful as a clue in tracking down past lives is as I've chanced upon such things as music or traditions or ideologies to which I'm drawn, and which don't match my present circumstances. In that respect, the important thing was the discrepancies between present expectations, and the unexpected longings or familiarity and feeling comfortable with things outside.