Predictions on ratios between the yield of light isotopes (Li, Be, B and C) from 112 Sn and 124 Sn nuclei from a microcanonical multifragmentation model and a sequential evaporation model are compared at excitation 175-800 MeV. Secondary decay does not affect these ratios significantly, which make them useful to estimate the excitation energy at the instant of fragmentation. The excitation energy dependence agrees well except for a local minimum near the onset of multifragmentation predicted only by the microcanonical model due to coupling between exit channels. This offers a way to distinguish between the models by measuring the relative yields of fragments from target residues differing significantly only in their neutron numbers.