Report Abuse. 1  Ripe Plum. 2  This Site Might Help You. 3  A plum is ripe when its soft and a really dark purpleish color.  they should be a little soft and should have a smell (plum smell) strawberry · 9 years 1  ago. Smell it, squish it and last but not least bite 2  it! plump(really soft) and dark all 3  over. the color tells u everything u need to  know.