SD rats were derived from the original Wistar line. They are both outbred rats and so differences are to be expected, even within strain. I have read that Wistar rats tend to have tail shorter than body length, while SD has a longer tail. I have never taken a look at tail length, so I cannot tell you if there is a measurable difference that could be used to tell one from the other.

Best way to tell the difference is to ensure that the animals are properly housed and identified upon arrival to your facility. The vendor would never ship more than one strain in a box, and should have safeguards in place to make sure that the correct strain is shipped. Your facility should has similar safeguards in place to ensure that all incoming animals are correctly tagged, housed and cage cards are accurate.