You can check it either hot or cold. The lines that correspond to 20C mean 20 degrees centigrade. The level should be there when cold. When it gets really hot after driving for a while, the 80C line should be used for the reading.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record