How do you diagnose overheating problems on a eclipse?

First thing you should check is the antifreeze level. If that is fine, replace the thermostat. If it still over heats after the thermostat, then most likely you need to replace the waterpump. If your antifreeze is low, check for leaks, the radiator, hoses, heater core, and water pump. If the water pump is leaking, that is your problem, replace that and thermostat all in one.