Colour and hardness, typically. Also, deformation (sagging). If there is (was) insulation, then whether or not the insulation is still there or not (whether or not the aluminum cladding melted, and whether or not the insulation burned off will also give indications of the temperature). Knowing the duration of the fire is very important.
During a fire event, it is critical to the recovery efforts to have video evidence of the event, showing the extent and duration of impingement. Stay out of the way of the first responders, and stay safe. But go to a safe location with a good vantage, and video record the fire, paying special attention to where the fire was and the impingement locations. Above certain temperatures, the colour of the steel won't change, so you may have to infer the temperature by other means - hence the video. And you're getting into the creep regime, so time at temperature is the important measure. Knowing whether equipment and piping was pressurized, or if the Emergency Shut Down (ESD) system depressurized it, is also important, because creep damage also needs stress at temperature and time.
