How can I tell if my yogurt is unsafe to eat?

Dannon and Stonyfield Farms both claim that "visible signs of mold" are the best way to determine if yogurt is unsafe to eat.  Live bacterial cultures in yogurt act as a preservative and keep mold at bay. Once those cultures start to die off, mold can start to form.   
 
If yogurt has been opened and refrigerated for more than 7-10 days or has gone beyond the range of the "best before" or "sell by" dates, it's not necessarily unsafe to eat. However, the quality begins to diminish as the bacteria die off. Sometimes mold is visible only as discoloration.  
 
Source(s): 
Dannon recorded consumer help line:  1-877-DANNONUS
Interview with Dannon consumer relations representative.  (name not recorded)
Stonyfield Farms recorded consumer help line:   1-800-PRO-COWS (776-2697)
Interview with Stonyfield Farms consumer relations representative. (name not recorded)