The DEF gauge (located on the instrument cluster) will display the level of DEF remaining in the tank INSTRUMENT CLUSTER DISPLAY.
Driving conditions (altitude, vehicle speed, load, etc.) will affect the amount of DEF that is used in your vehicle.
Another factor is that outside temperature can affect DEF consumption. In cold conditions, 12°F (-11°C) and below, the DEF gauge needle can stay on a fixed position and may not move for extended periods of time. This is a normal function of the system.
There is an electric heater inside the DEF tank that automatically works when necessary. If the DEF supply does freeze, the truck will operate normally until it thaws.