An electrical test like bypassing the thermostat (as suggested by applianceaid11) or a continuity test can be done depending on what you are comfortable with. If a continuity test would be preferable, unplug the refrigerator power cord, access the thermostat wires and disconnect one of them, then test the thermostat for continuity. If it does not have continuity while it is in the on position, and the temperature is too warm, then the thermostat has failed.