To determine if your probiotics are still alive, you can try the milk test. However, we will not comment on its reliability because it is a simple at-home test, and the results may not be accurate. For the milk test, add your probiotic supplement to at least 4 ounces of cold milk. Keep the glass of milk at room temperature for a day or two. Check the milk for any clump formations as time passes. If the milk curdles or takes on a yogurt-like consistency, it may mean the probiotics you purchased are viable.