https://www.livescience.com/65056-antimatter-matter-discovery-cern.html
Both of these mesons decay in many ways, but some small percentage of them end up as mesons called kaons or pions. The researchers measured the difference in decay rates between the D0 and the anti-D0 mesons, a process that involved taking indirect measurements to ensure they weren't just measuring a difference in the initial production of the two mesons, or differences in how well their equipment could detect various subatomic particles.
The bottom line? The ratios of decay differed by a tenth of a percent.