Established by the National Yogurt Association, the seal (shown here) indicates that the manufacturer is promising that the yogurt contains at least 100 million active starter cultures per gram when manufactured. It's not FDA-policed, though.
4. Get all the active cultures you're paying for.
Some yogurts are heat-treated after fermentation, which neutralizes the good-for-you bacteria required for production, meaning that the potential health benefits are neutralized, too. Check the packaging: The FDA mandates that these yogurts be labeled "heat-treated after culturing." If your yogurt is not heat-treated, the package may say "active yogurt cultures," "living yogurt cultures," or "contains active cultures."