I prefer using the term transformation range because it seems more definitive to me. This is the range of temperature at which austenite forms during heating and transforms during cooling. When the cooling is performed at a rapid rate, as in quenching, a martensitic structure is formed which is hardened steel. If you quench the steel before reaching this temperature unreliable results will occur. Accurately judging the temperature of the steel while forging and heat treating can be accomplished by a variety of methods.
Most smiths gauge the temperature by the color of the hot steel.
This method takes experience under the same repeated working conditions. This is unreliable for the beginner. Temperature color charts are popular in books on blacksmithing and knifemaking. Atmospheric conditions and ambient light can fool the eye, and opinions on what a specific color is called will differ. What you call sunrise red might be slightly different to another smith's eye.