ORIGINAL: npaden

One method that I've seen used is to have someone else loading the gun behind the shooter's back. He then hands the "loaded" gun to the shooter. Sometimes the gun has a cartridge in it, sometimes it doesn't. It becomes quickly apparent when you have a click on an empty chamber and the shooter flinches that you have an issue. Keep it up until the shooter doesn't flinch on an empty chamber. That means that they aren't flinching on the real thing either. It is a very effective method IME. Sometimes you don't realize that you are flinching until you try this.