Though I call this a "heat map" it could also be thought of as a "noise map". For each RGB pixel, I take the lowest RAW value and lookup a colour from a rainbow spectrum to illustrate the RAW value, which is inversely proportional to the amount of noise.
Areas that are blue, violet or black will have a significant amount of noise, particularly colour noise. Green to yellow to orange is "safe" and you want most of your image to be within this range. Dark reds are generally safe, but you might start having clipping or colour shifts in strong colours. There is a special strong yellow to indicate that you've almost certainly over-exposed one colour channel and white indicates that you've over-exposed all channels.