It's good to have a freezer thermometer that will provide information on the highest temperature recorded, allow alerts to be set and the amount of time that the temperature was above the alert level.
This is because you can lose power and your freezer can warm without you knowing it, other than to see physical signs that food had melted.
Thermoworks makes a basic thermometer that does this.
I had a new refrigerator that I bought that had an issue with its temperature controls. I notice ice cream had melted inside the freezer. The temperature was normal at the time.
I ended up putting a thermocouple in a cantaloupe in the refrigerator. The temperature was raising up to the 70's, inside the center of the cantaloupe, and then returning to normal refrigerator temperatures.
Imagine all the spoiled food I could have eaten.
I had to return the refrigerator because the service folks could not fix it.