C14 (<a href="https://en.wikipedia.org/wiki/Carbon-14" rel="nofollow">https://en.wikipedia.org/wiki/Carbon-14</a>). "Surface" carbon have a cycle where in some parts it gets into the high atmosphere, and that isotope is produced, and so that isotope is part of the "natural" carbon in the atmosphere and living things. But as that isotope is radioactive and degrade over a known period, long time buried carbon, the one in oil and coal, doesn't have it anymore.<p>Now, the carbon in the CO2 in the atmosphere have much less C14 on it that it should have, so can be calculated how much of it comes from fossil fuels.