I usually do this with a small light source (small enough to fit inside the bellows) and a very dark room. You'll see the pinholes pretty clearly and you can dab them with the liquid electrical tape goop pretty easily. I've found the stuff at Home Depot in the electrical department among the wires, boxes and breakers. It comes in both red and black, which is handy if you've got red bellows and you want to try to match the color.