Some years ago I was flying a UA commuter SFO-SMF on a Jetstream 31. I think this was back when the late FAT-based West Air (OE, I still remember their code) had the UAX contract on the West Coast. F/O completed his walkaround and all pre-flight inspections got on the plane, door was closed and we were ready for departure. I noticed that the overwing fuel cap was not replaced. I alerted the F/O and it was clear that had I not done so we would have taxied out without the fuel cap being closed. A call to ramp or maintenance got someone to come out and put the cap on but I will take credit for doing my little part to contribute to aviation safety!