Look at the shape and size of the peeper. Slender banana peppers grow 4 to 6 inches long with a pointed tip and a wider top; by contrast, pepperoncini peppers are shorter with a rounded tip. New Mexico chiles are wider in the middle, and yellow wax peppers are generally smoother.
Inspect the color. Under-ripe banana peppers are yellow-green, but turn a light yellow when ready for harvest. If left to fully ripen, they turn orange, then bright red, giving them an appearance similar to the hotter mirasol pepper.
Taste the pepper. Banana peppers are mild with a flavor that's more sweet than hot. If the flavor is hot, it is likely a yellow wax, pepperoncini, New Mexico chile or a mirasol.