The Antifreeze is part of the car engine cooling system system. We have 3 associated problem symptoms connected to it.
The engine temperature warning light shown in the picture (or one very similar depending on the car manufacturer) has appeared on the dashboard and stays on.
The warning light for maintenance has appeared on the dashboard. This light is different for different manufactures.
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.