1. ensure that a film holder sits flush to the back on all sides. small amounts of debris in the corners and light trap of the back will prevent the film holder from seating properly.
2. install the suspected faulty film holder, remove the lens board, and take the camera into total darkness.
3. shine a very bright light inside the camera and observe the exterior.
4. pull the darkslide on the camera side of the film holder.
5. anything other than totally black is a leak.
6. old school was a 40 watt appliance bulb in a small lamp. cfl's and led bulbs work. hot bulbs will burn the bellows if touched.