When I buy produce, I make sure that every item put in my bag is ripe and ready to use. An underripe plum is hard, tart, and lacking in the juicy sweetness we associate with plums. So when choosing a plum, be sure to remember the following:
Use your eyes: Color will not tell you if a plum is ripe in all varieties. My plum tree is a purple variety, and the plums are the exact same color when they are ripe as they were when they first began to grow. Lighter colored varieties of plums should be even in color and not look green...unless, of course, it is a green fleshed variety of plum.
Use your nose: Plums smell sweet and fruity when they are ripe. An underripe plum will have little to no scent at all.
Use your hands: A ripe plum is slightly soft with a smooth skin. Avoid hard, mushy, or wrinkled plums.