It depends on the timescale.
If you are asking about the past century, we can say with great certainty that human activity is responsible for ~100% of the observed rise in CO2. Dishonest pundits point to the fact that there are other natural sources and sinks of CO2. The biosphere and the oceans are enormous sources of CO2. Volcanoes are much less important. However, those sources were roughly in balance with natural sinks in the centuries preceding the industrial revolution. It only takes a small imbalance, compounded over decades, to account for the observed increase from pre-industrial levels of 280 ppm to modern levels of 385 ppm.
How do we know that this increase is due to humans? Because fossil fuel burning leaves an isotopic fingerprint in the atmosphere. Combustion also depletes the atmosphere of trace amounts of oxygen. We observe both of these features over the past century.