how did benjamin franklin discover that clouds are electrically charged in a thunder storm?

In a thunder charges from different clouds interact with each other and flow of charge can be see as thunder. So what Franklin did he put a kite in that cloud so the kites and its string gets charge as the charges on the cloud wanted to get discharge. When that passes through wet string it can produce an electric current. When Vranklin touched that string charges started to flow from Franklin body  since its voltage is very high its gave a shock to Franklin. This make him think about flow of current through material.