Even chiles of the same variety can range widely in heat; some jalapeños are mild and others spicy. But you don’t have to set your mouth on fire to find out how hot your chile is. Instead, cut it open and bring it close to your nose. If it’s hot you’ll immediately feel a tingling sensation, which can vary in intensity. If you sense no tingling, the chile is mild.
You have to cut the chile to determine its heat level because most of the heat is contained in the white, spongy mass of the core and the ribs. (It’s a misconception that the heat is in the seeds themselves.) If the tingling tells you that the chile is hotter than you like, you can use less of it or cut out the core and ribs. We prefer the former for small chiles because the hot parts also contain a lot of flavor.