Try reducing the manual Gain control (if fitted) to minimum setting (as against Auto/Cal/ or max). If the picture is still strongly red then avoid like a plague. This technique was usually successful when operating into Pacific islands where very heavy rain associated with towering Cu would cover the radar screen with red returns. By experiementing with the Gain control you could eliminate most of the heavy returns and the last one left was the one to avoid as it may be a CB rather than a towering Cu.
Same principle for high altitude cruise with certain types of radar. In this case set the Gain out of the automatic position into the MAX gain and the tops of CB normally invisible in AUTO gain, would show up as a very small echo. But enough to arouse suspicion that something big is in front. Tops of big CB up to 50,000 ft are usually dry and thus no reflection but with MAX gain often something shows up as a tiny return.
Get a copy of the radar manufacturer's Pilot Information Manual. Always good information in it.