FFB - the only caveat I would place on 'assessing' rain as opposed to a CB is to, as SC rightly says, use the tilt carefully. Just when you think it is 'OK ' to go through that red/green 'splodge' because it is rain, beware, and think where that rain comes from. Known always (well, nearly always) to travel downwards, remember the old advice of being careful if you fly UNDER a CB.
Looking at the radar 'contour' is normally reliable as stated.