An interesting look at the question, in - of all places - the Wall Street Journal:
It all began with a "serendipitous feeling" that hit him while he stumbled through Auschwitz in 2000. Like most visitors John Haedrich was deeply moved by what he saw. But this was something different. A kind of epiphany. Though raised a Christian, for reasons Mr. Haedrich cannot quite articulate, he began to suspect that he might be Jewish. Gradually, this hunch became too vital to ignore. He decided to investigate his origins by taking a DNA test, the results of which confirmed that he had, according to the test conclusion, "rather populous pedigree of Ashkenazi Polish Jews."Read the rest here. More background is here.