How do I know if my expressed milk is bad already?

Like what everyone says, smelling and tasting the milk is the best way to determine if the milk has gone bad. If it smell/taste sour, it is best to throw it out. Here are some useful Do's and Don'ts when it comes to storing milk: http://www.mayoclinic.org/healthy-lifestyle/infant-and-toddler-health/in-depth/breast-milk-storage/art-20046350