It's a process of elimination. The orbital/tilt factors say that the Earth should be getting cooler. The solar radiation levels are stable. It is a known fact that we are burning fossil fuels and cutting down rain forests, thereby releasing 7 gigatons of CO2 into the atmosphere each year. The atmospheric CO2 level goes up each year. The average global temperature is increasing.
I suppose that it depends upon the level of proof one requires to say that such and such is true.