What questions can you ask the customer to rule out operator error or bad ammunition?

With regard to the ammunition and whether or not it is the issue there are a few ways to determine the problem. First, I would ask the customer where they got the ammunition and how was it stored. This tells me if they bought it from the factory or if it has passed hands a few times before being fired. It also tells me if moisture has gotten to the ammunition causing it not to fire. It may be factory ammunition and they may store it correctly but if not bought from a reputable dealer but rather some guy at a show there is no guarantee that the previous owner stored the ammunition in a way that damaged it or not. Operator error is a different beast in itself. You may need to play 20 questions with someone to determine the actual issue because there are so many ways the user of the firearm may have attempted to use it incorrectly. It sounds silly but if someone tells me their firearm isn’t firing one of the first questions I have for them is did they chamber a round. It is too often someone will insert a magazine on a closed slide or action, and they tried to fire without chambering a round first. The very next questions I have are usually does their firearm have a safety, what kind, and did they disengage it. Sometimes the customer is so new to the firearm they cannot answer something as basic as this and that is okay. Remember, Ignorance is curable if you take the time to teach others what they may be doing wrong.