Apply gentle thumb pressure near the base of the stem. If it yields slightly, it's ripe. And you'll enjoy the full flavor and sweetness for which USA Pears are famous. You can't depend on color to tell you when a pear is ripe. Bartletts' will turn from green to yellow.
Some red varieties turn from dark red to bright red, but others remain dark red.
Anjou, Bosc, Comice and others, change very little from their unripened color, or not at all. Place your unripe USA Pears, of any variety, in a paper bag and let them stand at room temperature. Refrigerate them if you will not use them the same day.