Thermometers are good for several things but I have never used them while brooding. Different breeds and environmental factors make relying on a thermometer reading easy but not ideal. Since a picture is worth a thousand words please take time to read my three thousand ward essay on the subject.
Too cold: As you can see the chicks (black dots) are huddled under the heat (yellow circle). Avoiding water (red circle) and showing little interest in food (red rectangle). In this situation I would lower the heat source closer to the ground and check again in 5 minutes. Keep lowering it until the ideal temp is found.
Too hot: The chicks are trying to stay away from the heat source, hitting the water hard. Raise the heat source a little and keep checking every five minutes until the right temp is found.
Too drafty: Chicks are keeping to one side of brood area and staying close to the brooder shield. Light a match and find out where the draft originates.
Ideal brood temp: Chicks are evenly disbursed throughout brood area, eating and drinking normally. Pat yourself on the back, you are brooding like a professional.