An extensive study was conducted to determine isotopic ratios of nuclides in spent fuel that may be utilized to reveal historical characteristics of a nuclear reactor cycle. This forensic information is important to determine the origin of unknown nuclear waste. The distribution of isotopes in waste products provides information about a nuclear fuel cycle, even when the isotopes of uranium and plutonium are removed through chemical processing. Several different reactor cycles of the pressurized water reactor, boiling water reactor, Canada deuterium uranium reactor, and liquid-metal fast breeder reactor were simulated for this work with the ORIGEN-ARP and ORIGEN2.2 codes. The spent-fuel nuclide concentrations of these reactors were analyzed to find the most informative isotopic ratios indicative of irradiation cycle length and reactor design. Special focus was given to long-lived and stable fission products that would be present many years after their creation. For such nuclides, mass spectrometry analysis methods often have better detection limits than classic gamma-ray spectroscopy. The isotopic ratios 151Sm/146Sm, 149Sm/146Sm, and 244Cm/246Cm were found to be good indicators of fuel cycle length and are well suited for analysis by accelerator mass spectroscopy.