How can I check to see if the thermostat just needs liquid added? on 2004 Mercury Mountaineer

Test showed thermostat wasn't getting hot enough and they suggested adding liquid first after car cooled down. How do I check it and what type of liquid needs to be added? They didn't tell me that? 2004 Mercury Mountaineer

Asked by for the 2004 Mercury Mountaineer
Get someone who has more car savvy to give you a hand! Consult the owners manual.