Recycled bottle-grade poly(ethylene terephthalate), PET, can be mixed with virgin resin in different concentrations to be used by the food packaging industry. This will contribute to the growth of mechanical recycling of PET, but it is important to have an experimental method to detect the presence of recycled polymer in a batch. Differential scanning calorimetry was used to study the influences of the presence of post-consumption bottle-grade PET and of thermo-mechanical processing on the thermal properties of bottle-grade PET. When the virgin resin is subjected to thermo-mechanical processing (pre-forms, soft-drink bottles production, recycling or PET mixtures preparation), the crystallization rate is affected and a clearly defined crystallization peak, Tc, is observed. A bimodal melting temperature, Tm, behavior is observed for samples that were subjected to processing. When post-consumption PET is present in the mixtures, the Tm bimodal behavior is followed by a narrowing and a shift of the Tc to higher temperatures. Therefore, the crystallization rate, Tm and Tc are the principal thermal properties that can be used to distinguish between virgin and post-consumption resins submitted to thermo-mechanical processing.