As a rule of thumb, the smaller a mature pepper is, the more hot it's likely to be. Heat does vary from one pepper to the next in any given variety; you will learn to judge heat by the smell of the pepper as you work with them over time. Most of the heat is contained in the seeds and membrane; discarding all or part of those will greatly reduce the heat. Wear gloves when working with peppers, and wash your hands thoroughly with soap and hot water when you're done.