H has a charge of +1, and O has a charge of -2. In this case, C in C7H12 would have a charge of -12/7 to balance it out. Meanwhile, C in CO2 would have a charge of +4. Since the charge of C went from negative to positive, this implies that C lost electrons, which is indicative of oxidation (and thus it was the reducing agent). Therefore, C7H12 was the reducing agent, while O2 was the oxidizing agent.
