The cooling fan in the engine bay (usually next to the radiator) is very noisy, when it comes on you can hear it inside the cabin. The sound is often like a squeaking or whining.
The radiator fan is not starting up when it usually would, the engine can get very hot and the temperature gauge go up above normal levels but the fan still doesn't start up.
The temperature of the air coming from the vents is not as cold as it was before, it's not getting colder regardless of the setting.
The dashboard warning light showing on this page (or one very similar, sometimes a slightly different shape or colour) has appeared on the dashboard and doesn't go off when the car is running.
The engine temperature reading on the dashboard gauge is noticeably higher than usual. It doesn't seem to be caused by sitting in traffic on a hot day, it is generally just higher all the time.