Pay attention during the preflight. During oil changes and routine maintenance it is easy for the mechanics to forget stuff like tools. These tools pose a real risk to safety. As the Pilot in command it is your responsibility to make sure the aircraft is Airworthy and safe to fly. Guess whose fault it is if something like this where to cause an in flight emergency, yours or the mechanics? Hint CFR 91.3(a). Be focused and methodical with your preflight and don’t let distractions break your flow and miss something critical.