The phrase "definitive way" means the most reliable or final method or solution for something. It refers to an approach that is unquestionably correct or the best possible option. Full definition
Many healthcare practitioners and physicians believe that the only definitive way to identify the foods that are toxic to your body is by using an allergy avoidance diet and keeping a diary of symptoms. (whfoods.org)
Although these are not necessarily definitive ways to determine if your supply is lacking, they are signs that baby is not getting what they need. (momtricks.com)