On the CV-880 & CV-990 a warning light illuminated on the F/E annunciator panel if any gear pin was inserted. Pity Boeing and other manufacturers didn't follow this design, as I was caught out once with a 747F having been positioned on the ramp by maintenance with all main gear pins inserted without streamers on them. The last item on our after start checklist was all gear pins removed, which was confirmed by the departure engineer on the ground. After dumping 63 tons of fuel, we returned for a landing. The ground engineers said sorry but this incident still haunts me!