I had the same question with some Yo Baby yogurt just last month. I was given a couple of cases because it was close to the expiration date. I called Stoneyfield Farms (their "800" number should be on the packaging) and they told me not to worry. The date on the package is a sell-by date, not a use-by date. If properly stored it should be good for at least a couple of weeks after that date. The easiest way to tell? When you open up the yogurt, if it is bad it will have a little spot of mold growing on top. No mold means its fine. The customer service rep from Stoneyfield Farms even said that the mold wouldn't hurt you if you accidentally ate it, but that they don't reccomend it simply because it may adversely affect the taste.
Many brands put a customer service number on their products and its a great resource. Stoneyfield Farms even sent me coupons for free yogurt after calling them! Can't beat that!