How do you know if the thermostat needs to be changed?

Some automobile thermostats are designed to "fail open", which means that your temperature gauge will not rise above a "cold" indication. Failure can be caused by vibration (body cracking and allowing the coolant to freely circulate through the radiator), failure of wax substance bulb to expand and drive the valve closed against a spring that allows the valve to open when hot coolant is present. Other (usually older styles) thermostats "fail closed", preventing cooling from circulating through the radiator and overheating the engine. Advise: Purchase and install the most expensive quality thermostat available. These usually cost several dollars more, but the labor is 95% of the job. These jewels are usually buried under the tons of hoses, wires and do-dads added to the basic engine block. Good luck.