As mentioned, you really just look and smell. If there's no obvious mould growth and it smells like yogurt, it's good to go. These sorts of foods originated explicitly because they were a way to keep dairy products edible for a long(er) period of time. Like with sourdough, a lot of these fermenting bacteria keep food from being infected with spoilage microbes both by decreasing the pH and by simply being able to outcompete intruders.
A lot of these expiration dates are designed to be extra safe to both keep people happy and keep the sellers from having to deal with the issue.
