Only works if the parents know. I did the DNA test & came back with <1% - I had thought there was a chance I was because my GGGrandfather was from a town in Ukraine that was only a few miles from a Jewish town and there were people there with the same rare surname.
Then my German/Irish/Scot Mother took the test & returned 4% Jewish - one heck of a surprise to her. Nobody in the family knows where it would have come from. Her ancestors came over on the boat with the Wesley's and became big time Methodists because of it.