In my experience, the jalapeno pepper is the most variable of hot pepper varieties. I have bought jalapenos that were almost indistinguishable from bell peppers in heat and taste, and I’ve bought peppers that blew me away – from the same box in the grocery store! This is highly inconvenient. For stuffed jalapenos, I generally want something mildly hot so that my guests can enjoy 2 or 3 without pain. For salsa, I want something with some punch to it. Inevitably, I end up with the opposite of what I need – bland salsa and head-explodingly hot roasted stuffed peppers.
So, in all my experience working with jalapenos, I’ve only seen one tip that seems to work, most of the time. Look at the two peppers below:
You see how the one on the bottom has little brown lines forming in the skin, while the one at top is a uniform smooth green? Also, while it is hard to see in the photo, the pepper on the bottom also has a bit of color variation around the stem, a deep red / purple / green hue. That means it will be hotter than the top one.
How much hotter? How many Scoville units per brown line? Who knows…. it’s relative. But the one on the bottom will be hotter. Use this to select peppers at the market, and at least you can get an approximation of what you want. The smooth green peppers may be hot, but they won’t be as hot as the peppers with the veining.