You cannot reliably tell by looking whether a long green pepper is hot; peppers from the same market stall, or even the same plant, can be either mild or hot. The only dependable way is to taste it: cut a thin ring from the stem end and taste one small piece from near the inner rib. Since the heat is concentrated most in this area, a piece taken from the tip can make the pepper seem milder than it is.
A pepper’s heat comes from a substance called capsaicin, which is not distributed evenly throughout the pepper. The hottest part is the white inner rib where the seeds attach; this rib is most concentrated just below the stem. The flesh is less hot, and the tip is the mildest part. That is why biting the tip can be misleading: it may taste mild, while the second bite near the stem can burn. Where you take the tasting piece matters as much as the result itself.
Even peppers of the same variety can differ in heat. Variety is not the only factor; the conditions in which the plant grows also play a role. Peppers on plants that go without water during hot, dry days become hotter. Peppers from the same plant may differ between early and late summer. This is why a batch of peppers from the same market crate may include both mild and hot ones; even a well-meaning seller’s assurance that they are “not hot” may not apply to every pepper. So the taste of one pepper does not give certain information about the others in the same crate.
Some folk signs describe a hot pepper as thin and curved, with a thick stem or pointed tip, or fine lines on its skin. These clues are sometimes right, but they are not reliable measures: a smooth, large pepper may be hot, while a thin, curved one may be mild. Smell can tell you a little more: if cutting the pepper releases a sharp smell that stings your nose, it is probably hot. Still, tasting is what gives the final answer.
To taste a pepper, cut a thin ring from the stem end and place a small piece from near the ring’s inner rib on the tip of your tongue. You will feel the heat within a few seconds. You do not need to taste all the peppers in a 1 kg batch, but if you are preparing a dish such as salad or dolma where heat is unwanted, tasting a small piece from the stem end of each one can prevent surprises at the table. Keep your hands away from your face and eyes while handling hot peppers; wash your hands with soap when you are finished.
A pepper that turns out to be hot can still be used. Since most of the heat is in the inner rib and seeds, cutting the pepper in half and removing the rib and seeds makes it noticeably milder. If the remaining flesh is still hot, you can cook the pepper whole in the dish and remove it before serving; this transfers a little heat to the dish without leaving pepper in anyone’s bite. If you have used a hot pepper in a recipe that calls for a mild one, serving yogurt alongside the dish balances the heat. Set hot peppers aside in a separate container so they do not get mixed up with the others for the next meal.