I would assume they'd be a darker green, soft but still firm if you know what i mean, erm, there has to be a google answer, hang on:

Waheyyy, look at this, i was right


Pick cucumbers when they are dark green and firm. For slicing cucumbers, pick when they are six to eight inches long. Do not let cucumbers turn yellow. Check plants every other day for cucumbers that are ready to harvest. Cucumbers can grow from seed to harvest in 60 to 75 days.