Ripeness and maturity play a major role in the spice level of jalapeños. Not only do white stretch marks and spots appear
Overall, a jalapeño with a reddening exterior combined with white stretch marks or spots is likely to be hotter than the rest.
These stretch marks and spots are not just a sign of age, however. Often, these marks are a sign of stress endured by the peppers.
Stress can be caused by things like periods of dry soil between waterings, and that kind of stress combined with age can produce a spicier pepper.
Typically, jalapeños at grocery stores will be smooth and green, meaning that they will be somewhat milder and sweeter in flavor rather than spicy.
For those who only want a little heat from their peppers, this is perfect. However, for those who want peppers with more markings (and heat)