Some of my research was devoted to the division of competencies between the EU and the Member States. Powers transferred on the EU are defined in the respected articles of the Treaties. The implicit powers theory foresees that competences in external matters derive from explicit internal competence. In areas where the EU has neither explicit nor implicit powers, its powers are subsidiary (Article 352 of the Treaty on the Functioning of the EU). Judgements of the Court of Justice of the European Union may also contain statements on the matter of competencies of the European Commission.
Questions if a legal proposal is based on an appropriate legal base occupy the EU legislature as well as stakeholders constantly.
My experience gained in a highest national authority provided me with knowledge which can be applied to any new case of a dispute on competencies.