Important and crucial question you have asked.It is a broad issue required in depth knowledge on the subject right from planning of the study,identification of indicators,framing of survey instrument,data collection and analysis.In your case you may try Ex post impact assessment.You can consult literature of FAO, UNDP and world bank on methodological aspect.
Ex ante impact analysis. This is part of the needs analysis and planning activity of the policy cycle. It involves doing a prospective analysis of what the impact of an intervention might be, so as to inform policy making the policymaker’s equivalent of business planning;
Ex post impact assessment. This is part of the evaluation and management activity of the policy cycle. Broadly, evaluation aims to understand to what extent and how a policy intervention corrects the problem it was intended to address. Impact assessment focuses on the effects of the intervention, whereas evaluation is likely to cover a wider range of issues such as the appropriateness of the intervention design, the cost and efficiency of the intervention, its unintended effects and how to use the experience from this intervention to improve the design of future interventions.