According to USProbiotics.org, the federal government requires that food and supplement products be labeled in a truthful and not misleading fashion, but this is generally not enforced with regard to product content. FDA actions and warning letters seem to be issued mostly in the case of a food or supplement being labeled as a drug (i.e., the product claims to treat, cure, mitigate, diagnose or prevent disease), or if there is evidence of a safety concern. Although the FDA has not tested products for compliance with label claims, some watchdog groups and scientific researchers have. These published studies reveal many examples of commercial products that do not meet label claims for content. Therefore, it can be challenging to know which products to buy. Fortunately, this situation may improve with the FDA's recent ruling establishing regulations to require current good manufacturing practices for dietary supplements to be phased in over the next few years. Although these regulations don't address verification of efficacy claims, they will hopefully improve the compositional quality (identity, purity, and strength) of probiotic supplements on the U.S. market (you can read more here.)