In his initial work, Robert Putnam proposed measuring social capital using the following types of statistics: membership of voluntary groups or the number of such groups, voter turnout and reading of magazines. These indicators are designed to show the attitude of people indicating the presence of high or low social capital. High social capital means that many people will associate in voluntary groups for a variety of common and individual purposes, have actively participated in public life by participating in the election, and have read magazines readily. He proposed to measure the level of altruism (understood as doing good other people), as measured by the share of charitable spending in relation to overall expenditure made by individuals.

Another proposal is to measure the number of blood donors. Altruism can be the right measure for social capital because of strong relationship between people's involvement in society and altruism.

It depends a little on what you want to find out and what conception of social capital you adopt: generalised trust (eg Granovetter); trust and reciprocity (eg Putnam); or networks through which social resources flow (eg Burt, Lin). Many approaches to social capital research measure levels of social capital, but relatively few measure how it is mobilised. Lin's work and those who have combined social network analysis with social capital analysis probably come closest to this. 

Some of the relevant readings to your inquiry. 

http://www.emeraldinsight.com/doi/full/10.1108/IJEBR-04-2015-0090

http://www.emeraldinsight.com/doi/full/10.1108/IJEBR-07-2015-0147

http://www.management-aims.com/fichiers/publications/en_1430934252.pdf

http://onlinelibrary.wiley.com/doi/10.1111/fare.12017/abstract

http://www.e-elgar.com/shop/family-business-and-social-capital