A method based on the analysis of the temperature sensor of a high-resolution AC calorimeter has been developed in order to determine the character of the phase transition in liquid crystals. A comparative study between this method, denoted as 'thermistor method', and the standard phase shift procedure is carried out in different kind of liquid crystal phase transitions. The sensitivity of the standard phase shift method has been studied through the thermal models for our system. Finally, the method, when it is applied to first-order transitions, provides additional thermal features as coexistence region, metastability and latent heat ratio between transitions, and it could allow to elucidate the assignment of the character of the transition lines in phase diagrams.