Buyer beware! Always read the ingredients. Look for casein and whey specifically. Sometimes you can also look at the allergens list. However, member Scott Fulton pointed out that inclusion of casein may or may not be noted as milk on the allergen list.
“Depends if the manufacturer deems it [casein] an ingredient or a processing agent. Casein is a more shelf stable protein, making it qualify for the latter. There is no FDA audit process – presumption is producer is abiding by rules and subject to penalty only upon proven non-compliance.”