Impact of what? I don't have enough information to give an answer. Whose Impact, what factors. SC can be measured through trust in colleagues for example. But what secondary data you have or want to have is unclear. It is necessary to define what is secondary data. You need to look for indicators of measurement. In an organization, you can trust work information, give access to the workplace. Look for indicators, these should be indicators by which you can judge the trust of colleagues in each other.
Social capital is a concept that is created by the interaction of individual interest with social interest. You can assess the impact of social capital by studying the actual beneficiaries who are directly benefited from the organisation through primary data interpretation.