Theories abound as for how to determine just how hot a jalapeño is: Some claim that smaller jalapeños are spicier than their larger ilk. Others say that a pepper with a pointy tip brings the heat. While the smaller peppers theory more or less holds true when comparing varieties of peppers — think of the heat and size correlation between bell, jalapeño, serrano, and Thai bird chiles — from jalapeño to jalapeño, not so much. As for pointy-tipped peppers: this seems more old wives' tale than truth. Visual cues related to the age of a jalapeño, such as white striations on the pepper (they look almost like stretch marks) or a red hue (jalapeños change from green to red as they mature), can signal a hotter pepper, but we've also had our fair share of shiny, smooth green jalapeños that set our mouths on fire. In our experience there's only one surefire way to tell: taste it.
This might seem like a cop-out answer, but hear us out. The next time you set out to use a jalapeño in a recipe — guacamole mason-jar salad anyone? — slice off a snippet of the pepper from the stem end (which you'd be removing for most recipes anyway) and give it a taste. This way you'll know exactly how hot that pepper is, and you can adjust your recipe accordingly, either by removing the membrane and seeds (if it's too hot for your liking), or adding an extra pepper or two to the mix (if it's too mild for your taste).