Questions
"What would convince you that global warming is mostly a result of human activities?"
Do you mean that the increase of global temperatures since the end of the Little Ice Age and that this is mostly or some significant portion due to the sum total of all human activity?
Or perhaps you are referring the increase in global temperature since the short cooling period [late 40s thru 1970s] and are asking if this is caused by human emission of CO2 or CO2 and other factors.
Or are you asking about the most recent global warming period [starting +10,000 years ago] and that this was mostly a result of human activities?