Look at the cucumber. For the cultivars most Americans are familiar with -- regular cucumbers, English cucumbers and pickling cucumbers -- the skin should be medium to dark green. Yellow streaks or a yellow tinge indicate that it is overripe.
Pick the cucumber up. Pay attention to its shape. If it is fatter in the middle than near the ends it's full of seeds and the flesh probably doesn't have much flavor. Though a 5-inch cucumber can be just ripe as a 15-inch cucumber, keep in mind that the smaller it is, the fewer seeds it will have. Make sure the skin is smooth and intact, and doesn't have any wrinkles.
Squeeze the cucumber lightly. A ripe cucumber will be firm to the touch. Make sure there aren't any mushy spots, as this can indicate that it is overripe.