There are no definitive ways to tell for certain if you have Jewish ancestry based on your DNA. However there are a few ways you can determine whether the possibility exists.
First, you may have evidence of Ashkenazi ancestry in your Ancestry Composition results. DNA shows clearly the connections among those who consider themselves to be Ashkenazi Jewish: two Ashkenazi Jewish people are very likely to be "genetic cousins," sharing long stretches of identical DNA. This sharing reflects the close knit nature of this population.
Another source is your haplogroup. There have been several well-supported studies chronicling the genetic ancestry of Ashkenazi Jews, which we do report in the Maternal Line summaries. The maternal haplogroups associated with Ashkenazi ancestry are N1b, K1a1b1a, K1a9, and K2a2a. If you fall into one of these haplogroups, it is still possible that you are not Jewish, but it is probable that you are.
The final way to determine whether you may have Jewish ancestry is through the Countries of Ancestry tool. You can get there by selecting "Ancestry Tools" from the My Results menu. In this feature we combine information from your DNA Relatives (regardless of whether you have made contact or revealed your identities to one another) and those matches' answers to the Where Are You From? ancestry survey. The survey allows for people to self-identify as having Ashkenazi Jewish ancestry.
Using the feature's advanced controls, you can check a box indicating segments declared to be of Ashkenazi Jewish ancestry. In this way, you can see whether or not and to what degree you have DNA Relatives who consider themselves Ashkenazi Jewish. From this you can infer whether or not and to what degree you may have Jewish ancestry.