How does one tell if a jalapeno is spicy?
The only way to test for capsaicin besides tasting is chemical testing, namely liquid chromatography. Problem is, the same kind of chili can be quite different in heat, even on the same bush it can be quite different from my experience. No idea why, though.

Here is one tip on how to check the heat of a chili without actually eating part of it, but still cutting it apart: Cut through the chili below the stem, touch the membranes, touch your tongues, feel the heat.
http://www.chow.com/videos/show/chow-tips/78517/how-to-test-the-heat-of-your-chiles

You can adjust the amount of heat a chili adds to the dish by removing the white membranes. Just add the colored flesh, check for heat, and add membranes until the desired spiciness is obtained. Capsaicin is only produced in the white mebranes in the center. It may move inside the fruit, though. Areas closer to the membranes will be hotter (like, the seeds), than other parts.