1. A method of determining the sufficiency of refrigerant charge in a refrigeration system having a compressor, a condenser, an expansion device and an evaporator fluidly connected in serial refrigerant flow relationship comprising the steps of: sensing the temperature of the liquid refrigerant leaving the condenser; sensing a second temperature which, if said liquid refrigerant temperature is subtracted therefrom, will provide an indication of the refrigerant charge sufficiency in the system; and visually displaying by way of thermochromic strips both said sensed liquid refrigerant temperature and said sensed second temperature such that the respective position of each display is representative of its value, and the relative positions of the sensed temperature displays are indicative of refrigerant charge sufficiency in the system.