The best way to measure the success of a leadership coaching is to ask the participants. This can be done through short and simple valuations to more detailed structured questionnaires. Attempts to construct a coaching ROI (return on investment) often times do not work in practice. However, it's still possible to construct separate KPI when this is required. If that's the case it's important to agree on KPIs beforehand to avoid any conflicting expectations and to collect reasonable metrics.